L3, a novel murine LIM-homeodomain transcription factor expressed in the ventral telencephalon and the mesenchyme surrounding the oral cavity

Genetic Regulation of Vertebrate Forebrain Development by Homeobox Genes

Forebrain development in vertebrates is regulated by transcription factors encoded by homeobox, bHLH and forkhead gene families throughout the progressive and overlapping stages of neural induction and patterning, regional specification and generation of neurons and glia from central nervous system (CNS) progenitor cells. Moreover, cell fate decisions, differentiation and migration of these committed CNS progenitors are controlled by the gene regulatory networks that are regulated by various homeodomain-containing transcription factors, including but not limited to those of the Pax (paired), Nkx, Otx (orthodenticle), Gsx/Gsh (genetic screened), and Dlx (distal-less) homeobox gene families. This comprehensive review outlines the integral role of key homeobox transcription factors and their target genes on forebrain development, focused primarily on the telencephalon. Furthermore, links of these transcription factors to human diseases, such as neurodevelopmental disorders and brain tumors are provided.

LIM homeodomain proteins and associated partners: Then and now

The field of molecular embryology started around 1990 by identifying new genes and analyzing their functions in early vertebrate embryogenesis. Those genes encode transcription factors, signaling molecules, their regulators, etc. Most of those genes are relatively highly expressed in specific regions or exhibit dramatic phenotypes when ectopically expressed or mutated. This review focuses on one of those genes, Lim1/Lhx1, which encodes a transcription factor. Lim1/Lhx1 is a member of the LIM homeodomain (LIM-HD) protein family, and its intimate partner, Ldb1/NLI, binds to two tandem LIM domains of LIM-HDs. The most ancient LIM-HD protein and its partnership with Ldb1 were innovated in the metazoan ancestor by gene fusion combining LIM domains and a homeodomain and by creating the LIM domain-interacting domain (LID) in ancestral Ldb, respectively. The LIM domain has multiple interacting interphases, and Ldb1 has a dimerization domain (DD), the LID, and other interacting domains that bind to Ssbp2/3/4 and the boundary factor, CTCF. By means of these domains, LIM-HD-Ldb1 functions as a hub protein complex, enabling more intricate and elaborate gene regulation. The common, ancestral role of LIM-HD proteins is neuron cell-type specification. Additionally, Lim1/Lhx1 serves crucial roles in the gastrula organizer and in kidney development. Recent studies using Xenopus embryos have revealed Lim1/Lhx1 functions and regulatory mechanisms during development and regeneration, providing insight into evolutionary developmental biology, functional genomics, gene regulatory networks, and regenerative medicine. In this review, we also discuss recent progress at unraveling participation of Ldb1, Ssbp, and CTCF in enhanceosomes, long-distance enhancer-promoter interactions, and trans-interactions between chromosomes.

FGF8 and BMP2 mediated dynamic regulation of dental mesenchyme proliferation and differentiation via Lhx8/Suv39h1 complex

The homeobox gene, LIM-homeobox 8 (Lhx8), has previously been identified as an essential transcription factor for dental mesenchymal development. However, how Lhx8 itself is regulated and regulates odontogenesis remains poorly understood. In this study, we employed an RNAscope assay to detect the co-expression pattern of Lhx8 and Suv39h1 in the dental mesenchyme, which coincided with the dynamic expression profiles of the early epithelium signal of Fibroblast Growth Factor 8 (FGF8) and the later mesenchymal signal Bone Morphogenetic Protein 2 (BMP2). Moreover, FGF8 activated Lhx8, whereas BMP2 repressed Lhx8 expression at the transcriptional level. The high expression of Lhx8 in the early dental mesenchyme maintained the cell fate in an undifferentiated status by interacting with Suv39h1, a histone-lysine N-methyltransferase constitutively expressed in the dental mesenchyme. Further in the ex vivo organ culture model, the knockdown of Suv39h1 significantly blocked the function of Lhx8 and FGF8. Mechanistically, Lhx8/Suv39h1 recognized the odontoblast differentiation-related genes and repressed gene expression via methylating H3K9 on their promoters. Taken together, our data here suggest that Lhx8/Suv39h1 complex is inversely regulated by epithelium-mesenchymal signals, balancing the differentiation and proliferation of dental mesenchyme via H3K9 methylation.

Aging-relevant human basal forebrain cholinergic neurons as a cell model for Alzheimer's disease

Background

Alzheimer’s disease (AD) is an adult-onset mental disorder with aging as a major risk factor. Early and progressive degeneration of basal forebrain cholinergic neurons (BFCNs) contributes substantially to cognitive impairments of AD. An aging-relevant cell model of BFCNs will critically help understand AD and identify potential therapeutics. Recent studies demonstrate that induced neurons directly reprogrammed from adult human skin fibroblasts retain aging-associated features. However, human induced BFCNs (hiBFCNs) have yet to be achieved.

Methods

We examined a reprogramming procedure for the generation of aging-relevant hiBFCNs through virus-mediated expression of fate-determining transcription factors. Skin fibroblasts were obtained from healthy young persons, healthy adults and sporadic AD patients. Properties of the induced neurons were examined by immunocytochemistry, qRT-PCR, western blotting, and electrophysiology.

Results

We established a protocol for efficient generation of hiBFCNs from adult human skin fibroblasts. They show electrophysiological properties of mature neurons and express BFCN-specific markers, such as CHAT, p75NTR, ISL1, and VACHT. As a proof-of-concept, our preliminary results further reveal that hiBFCNs from sporadic AD patients exhibit time-dependent TAU hyperphosphorylation in the soma and dysfunctional nucleocytoplasmic transport activities.

Conclusions

Aging-relevant BFCNs can be directly reprogrammed from human skin fibroblasts of healthy adults and sporadic AD patients. They show promises as an aging-relevant cell model for understanding AD pathology and may be employed for therapeutics identification for AD.

USP1 inhibitor ML323 enhances osteogenic potential of human dental pulp stem cells

LIM homeobox 8 (LHX8) is expressed during embryonic development of craniofacial tissues, including bone and teeth. In a previous study, the overexpression of LHX8 inhibited osteodifferentiation of human dental pulp stem cells (DPSCs). In this study, a cDNA microarray analysis was performed to reveal the molecular changes which occur in response to LHX8 overexpression in DPSCs and discover possible targets for an osteoinductive agent. There were 345 differentially expressed genes (DEGs) in response to osteoinductive signaling and 53 DEGs in response to LHX8 overexpression and osteoinductive signaling, respectively. Thirty-eight genes were common in both conditions, and among these, genes upregulated in LHX8 DPSCs but downregulated in osteodifferentiated DPSCs were chosen. Five of them had commercial inhibitors available. Among the tested inhibitors, ML323, which target DNA-binding protein inhibitor ID-1, promoted osteodifferentiation of DPSCs. In conclusion, inhibition of ID-1 led to increased osteogenesis of human DPSCs.

Optogenetic control of mesenchymal cell fate towards precise bone regeneration

Rationale: Spatial-temporal control of cell fate in vivo is of great importance for regenerative medicine. Currently, there remain no practical strategies to tune cell-fate spatial-temporally. Optogenetics is a biological technique that widely used to control cell activity in genetically defined neurons in a spatiotemporal-specific manner by light. In this study, optogenetics was repurposed for precise bone tissue regeneration.

Methods: Lhx8 and BMP2 genes, which are considered as the master genes for mesenchymal stem cell proliferation and differentiation respectively, were recombined into a customized optogenetic control system. In the system, Lhx8 was constitutively expressed, while BMP2 together with shLhx8 expression was driven by blue light.

Results: As expected, blue light induced BMP2 expression and inactivated Lhx8 expression in cells infected with the optogenetic control system. Optogenetic control of BMP2 and Lhx8 expression inversely regulates MSC fate in vitro. By animal study, we found that blue light could fine-tune the regeneration in vivo. Blue light illumination significantly promotes bone regeneration when the scaffold was loaded with MSCs infected with adeno-Lhx8, GI-Gal4DBD, LOV-VP16, and BMP2-shLhx8.

Conclusions: Together, our study revealed that optogenetic control of the master genes for mesenchymal stem cell proliferation and differentiation would be such a candidate strategy for precise regenerative medicine.

Direct conversion of mouse astrocytes into neural progenitor cells and specific lineages of neurons

Background

Cell replacement therapy has been envisioned as a promising treatment for neurodegenerative diseases. Due to the ethical concerns of ESCs-derived neural progenitor cells (NPCs) and tumorigenic potential of iPSCs, reprogramming of somatic cells directly into multipotent NPCs has emerged as a preferred approach for cell transplantation.

Methods

Mouse astrocytes were reprogrammed into NPCs by the overexpression of transcription factors (TFs) Foxg1, Sox2, and Brn2. The generation of subtypes of neurons was directed by the force expression of cell-type specific TFs Lhx8 or Foxa2/Lmx1a.

Results

Astrocyte-derived induced NPCs (AiNPCs) share high similarities, including the expression of NPC-specific genes, DNA methylation patterns, the ability to proliferate and differentiate, with the wild type NPCs. The AiNPCs are committed to the forebrain identity and predominantly differentiated into glutamatergic and GABAergic neuronal subtypes. Interestingly, additional overexpression of TFs Lhx8 and Foxa2/Lmx1a in AiNPCs promoted cholinergic and dopaminergic neuronal differentiation, respectively.

Conclusions

Our studies suggest that astrocytes can be converted into AiNPCs and lineage-committed AiNPCs can acquire differentiation potential of other lineages through forced expression of specific TFs. Understanding the impact of the TF sets on the reprogramming and differentiation into specific lineages of neurons will provide valuable strategies for astrocyte-based cell therapy in neurodegenerative diseases.

Homeobox genes and tooth development: Understanding the biological pathways and applications in regenerative dental science

OBJECTIVES

Homeobox genes are a group of conserved class of transcription factors that function as key regulators during the embryonic developmental processes. They act as master regulator for developmental genes, which involves coordinated actions of various auto and cross-regulatory mechanisms. In this review, we summarize the expression pattern of homeobox genes in relation to the tooth development and various signaling pathways or molecules contributing to the specific actions of these genes in the regulation of odontogenesis.

MATERIALS AND METHODS

An electronic search was undertaken using combination of keywords e.g. Homeobox genes, tooth development, dental diseases, stem cells, induced pluripotent stem cells, gene control region was used as search terms in PubMed and Web of Science and relevant full text articles and abstract were retrieved that were written in English. A manual hand search in text books were also carried out. Articles related to homeobox genes in dentistry and tissue engineering and regenerative medicine of odontogenesis were selected.

RESULTS

The possible perspective of stem cells technology in odontogenesis and subsequent analysis of gene correction pertaining to dental disorders through the possibility of induced pluripotent stem cells technology is also inferred.

CONCLUSIONS

We demonstrate the promising role of tissue engineering and regenerative medicine on odontogenesis, which can generate a new ray of hope in the field of dental science.

Regulation and/or Repression of Cholinergic Differentiation of Murine Embryonic Stem Cells Using RNAi Directed Against Transcription Factor L3/Lhx8

Techniques for controlling the expression of a specific gene in embryonic stem cells are effective and important for clarifying the functions of the gene. Regarding differentiation of cells into nervous system components, these techniques would play key roles in elucidating, not only the differentiation mechanisms of neuronal and glial cells but also how neuronal phenotypes are determined. In this chapter, we describe a RNA interference method for suppressing cholinergic differentiation in murine embryonic stem cells by knockdown of expression of the transcription factor L3/Lhx8, a Lim homeobox gene family protein. This method will greatly facilitate functional analyses of the factors involved in neuronal differentiation and regeneration and will contribute to cell transplantation studies.

Generation of Cholinergic and Dopaminergic Interneurons from Human Pluripotent Stem Cells as a Relevant Tool for In Vitro Modeling of Neurological Disorders Pathology and Therapy

The cellular and molecular bases of neurological diseases have been studied for decades; however, the underlying mechanisms are not yet fully elucidated. Compared with other disorders, diseases of the nervous system have been very difficult to study mainly due to the inaccessibility of the human brain and live neurons in vivo or in vitro and difficulties in examination of human postmortem brain tissue. Despite the availability of various genetically engineered animal models, these systems are still not adequate enough due to species variation and differences in genetic background. Human induced pluripotent stem cells (hiPSCs) reprogrammed from patient somatic cells possess the potential to differentiate into any cell type, including neural progenitor cells and postmitotic neurons; thus, they open a new area to in vitro modeling of neurological diseases and their potential treatment. Currently, many protocols for generation of various neuronal subtypes are being developed; however, most of them still require further optimization. Here, we highlight accomplishments made in the generation of dopaminergic and cholinergic neurons, the two subtypes most affected in Alzheimer's and Parkinson's diseases and indirectly affected in Huntington's disease. Furthermore, we discuss the potential role of hiPSC-derived neurons in the modeling and treatment of neurological diseases related to dopaminergic and cholinergic system dysfunction.

Common pitfalls of stem cell differentiation: a guide to improving protocols for neurodegenerative disease models and research

Induced pluripotent stem cells and embryonic stem cells have revolutionized cellular neuroscience, providing the opportunity to model neurological diseases and test potential therapeutics in a pre-clinical setting. The power of these models has been widely discussed, but the potential pitfalls of stem cell differentiation in this research are less well described. We have analyzed the literature that describes differentiation of human pluripotent stem cells into three neural cell types that are commonly used to study diseases, including forebrain cholinergic neurons for Alzheimer's disease, midbrain dopaminergic neurons for Parkinson's disease and cortical astrocytes for neurodegenerative and psychiatric disorders. Published protocols for differentiation vary widely in the reported efficiency of target cell generation. Additionally, characterization of the cells by expression profile and functionality differs between studies and is often insufficient, leading to highly variable protocol outcomes. We have synthesized this information into a simple methodology that can be followed when performing or assessing differentiation techniques. Finally we propose three considerations for future research, including the use of physiological O2 conditions, three-dimensional co-culture systems and microfluidics to control feeding cycles and growth factor gradients. Following these guidelines will help researchers to ensure that robust and meaningful data is generated, enabling the full potential of stem cell differentiation for disease modeling and regenerative medicine.

Lhx6 and Lhx8: cell fate regulators and beyond

As transcription factors of the lines (LIN)-11/Islet (Isl)-1/mitosis entry checkpoint (MEC)-3 (LIM)-homeobox subfamily, LIM homeobox (Lhx)6 and -8 are remarkably conserved and involved in the morphogenesis of multiple organ systems. Lhx6 and -8 play overlapping and distinctive roles, but in general act as cell fate mediators and in turn are regulated by several transcriptional factors, such as sonic hedgehog, fibroblast growth factors, and wingless-int (Wnt)/β-catenin. In this review, we first summarize Lhx6 and -8 distributions in development and then explore how Lhx6 and -8 act as transcription factors and coregulators of cell lineage specification. Known Lhx6 and -8 functions and targets are outlined in neurogenesis, craniofacial development, and germ cell differentiation. The underlying mechanisms of Lhx6 and -8 in regulating cell fate remain elusive. Whether Lhx6 and -8 affect functions in tissues and organs other than neural, craniofacial, oocytes, and germ cells is largely unexplored. Taken together, Lhx6 and -8 are important regulators of cell lineage specification and may act as one of the pivotal mediators of stem cell fate. Undoubtedly, future investigations of Lhx6 and -8 biology will continue to yield fascinating insights into tissue development and homeostasis, in addition to their putative roles in tissue regeneration and ageing.

Lhx8 mediated Wnt and TGFβ pathways in tooth development and regeneration

LIM homeobox 8 (Lhx8) is a highly conserved transcriptional factor with recently illustrated roles in cholinergic and GABAergic differentiation, and is expressed in neural crest derived craniofacial tissues during development. However, Lhx8 functions and signaling pathways are largely elusive. Here we showed that Lhx8 regulates dental mesenchyme differentiation and function via Wnt and TGFβ pathways. Lhx8 expression was restricted to dental mesenchyme from E11.5 to a peak at E14.5, and absent in dental epithelium. By reconstituting dental epithelium and mesenchyme in an E16.5 tooth organ, Lhx8 knockdown accelerated dental mesenchyme differentiation; conversely, Lhx8 overexpression attenuated dentin formation. Lhx8 overexpressed adult human dental pulp stem/progenitor cells in β-tricalcium phosphate cubes attenuated mineralized matrix production in vivo. Gene profiling revealed that postnatal dental pulp stem/progenitor cells upon Lhx8 overexpression modified matrix related gene expression including Dspp, Cola1 and osteocalcin. Lhx8 transcriptionally activated Wnt and TGFβ pathways, and its attenuation upregulated multiple dentinogenesis genes. Together, Lhx8 regulates dentin development and regeneration by fine-turning Wnt and TGFβ signaling.

Identification of a face enhancer reveals direct regulation of LIM homeobox 8 (Lhx8) by wingless-int (WNT)/β-catenin signaling

Development of the mammalian face requires a large number of genes that are expressed with spatio-temporal specificity, and transcriptional regulation mediated by enhancers plays a key role in the precise control of gene expression. Using chromatin immunoprecipitation for a histone marker of active enhancers, we generated a genome-wide map of candidate enhancers from the maxillary arch (primordium for the upper jaw) of mouse embryos. Furthermore, we confirmed multiple novel craniofacial enhancers near the genes implicated in human palate defects through functional assays. We characterized in detail one of the enhancers (Lhx8_enh1) located upstream of Lhx8, a key regulatory gene for craniofacial development. Lhx8_enh1 contained an evolutionarily conserved binding site for lymphoid enhancer factor/T-cell factor family proteins, which mediate the transcriptional regulation by the WNT/β-catenin signaling pathway. We demonstrated in vitro that WNT/β-catenin signaling was indeed essential for the expression of Lhx8 in the maxillary arch cells and that Lhx8_enh1 was a direct target of the WNT/β-catenin pathway. Together, we uncovered a molecular mechanism for the regulation of Lhx8, and we provided valuable resources for further investigation into the gene regulatory network of craniofacial development.

WNK signaling is involved in neural development via Lhx8/Awh expression

WNK kinase family is conserved among many species and regulates SPAK/OSR1 and ion co-transporters. Some mutations in human WNK1 or WNK4 are associated with Pseudohypoaldosteronism type II, a form of hypertension. WNK is also involved in developmental and cellular processes, but the molecular mechanisms underlying its regulation in these processes remain unknown. Here, we identify a new target gene in WNK signaling, Arrowhead and Lhx8, which is a mammalian homologue of Drosophila Arrowhead. In Drosophila, WNK was shown to genetically interact with Arrowhead. In Wnk1 knockout mice, levels of Lhx8 expression were reduced. Ectopic expression of WNK1, WNK4 or Osr1 in mammalian cells induced the expression of the Lhx8. Moreover, neural specification was inhibited by the knockdown of both Wnk1 and Wnk4 or Lhx8. Drosophila WNK mutant caused defects in axon guidance during embryogenesis. These results suggest that WNK signaling is involved in the morphological and neural development via Lhx8/Arrowhead.

Lhx8 promote differentiation of hippocampal neural stem/progenitor cells into cholinergic neurons in vitro

Lhx8, also named L3, is a recently identified member of the LIM homeobox gene family. Previously, we found acetylcholinesterase (AChE)-positive cells in fimbria-fornix (FF) transected rat hippocampal subgranular zone (SGZ). In the present study, we detected choline acetyltransferase (ChAT)-positive cholinergic cells in hippocampal SGZ after FF transaction, and these ChAT-positive cells were double labeled by Lhx8. Then we overexpressed Lhx8 during neural differentiation of hippocampal neural stem/progenitor cells on adherent conditions using lentivirus Lenti6.3-Lhx8. The result indicated that overexpression of Lhx8 did not affect the proportion of MAP2-positive neurons, but increased the proportion of ChAT-positive cells in vitro. These results suggested that FF-transected hippocampal niche promoted the ChAT/Lhx8-positive cholinergic neurons generation in rodent hippocampus, and Lhx8 was not associated with the MAP2-positive neurons differentiation on adherent conditions, but played a role in the specification of cholinergic neurons derived from hippocampal neural stem/progenitor cells in vitro.

Evaluation of the LIM homeobox genes LHX6 and LHX8 as candidates for Tourette syndrome

The etiology and pathophysiology of Tourette Syndrome (TS) remain poorly understood. Multiple lines of evidence suggest that a complex genetic background and the cortico-striato-thalamo-cortical circuit are involved. The role of Lhx6 and Lhx8 in the development of the striatal interneurons, prompted us to investigate them as novel candidate genes for TS. We performed a comparative study of the expression of Lhx6 and Lhx8 and investigated genetic association with TS using two samples of trios (TSGeneSEE and German sample - 222 families). We show that Lhx6 and Lhx8 expression in the forebrain is evolutionarily conserved, underlining their possible importance in TS-related pathophysiological pathways. Our tagging-single nucleotide polymorphism (tSNP)-based association analysis was negative for association with LHX8. However, we found positive association with LHX6 in the TSGeneSEE sample (corrected P-value = 0.006 for three-site haplotype around SNP rs3808901) but no association in the sample of German families. Interestingly, the SNP allele that was identified to be significantly associated in the TSGeneSEE dataset, showed an opposite trend of transmission in the German dataset. Our analysis of the correlation of the LHX6 region with individual ancestry within Europe, revealed the fact that this particular SNP demonstrates a high degree of population differentiation and is correlated with the North to South axis of European genetic variation. Our results indicate that further study of the LHX6 gene in relation to the TS phenotype is warranted and suggest the intriguing hypothesis that different genetic factors may contribute to the etiology of TS in different populations, even within Europe.

The controlled generation of functional basal forebrain cholinergic neurons from human embryonic stem cells

An early substantial loss of basal forebrain cholinergic neurons (BFCN) is a constant feature of Alzheimer's disease and is associated with deficits in spatial learning and memory. The ability to selectively control the differentiation of human embryonic stem cells (hESCs) into BFCN would be a significant step toward a cell replacement therapy. We demonstrate here a method for the derivation of a predominantly pure population of BFCN from hESC cells using diffusible ligands present in the forebrain at developmentally relevant time periods. Overexpression of two relevant human transcription factors in hESC-derived neural progenitors also generates BFCN. These neurons express only those markers characteristic of BFCN, generate action potentials, and form functional cholinergic synapses in murine hippocampal slice cultures. siRNA-mediated knockdown of the transcription factors blocks BFCN generation by the diffusible ligands, clearly demonstrating the factors both necessary and sufficient for the controlled derivation of this neuronal population. The ability to selectively control the differentiation of hESCs into BFCN is a significant step both for understanding mechanisms regulating BFCN lineage commitment and for the development of both cell transplant-mediated therapeutic interventions for Alzheimer's disease and high-throughput screening for agents that promote BFCN survival.

Comparative study of LHX8 expression between odontoma and dental tissue-derived stem cells

BACKGROUND

LHX8 (LIM-homeobox gene 8) is known as an important regulating factor in tooth morphogenesis. Odontoma is a mixed odontogenic tumor where epithelium and mesenchyme differentiated together, resulting in anomalous tooth structures. In this study, gene and protein expressions of LHX8 were analyzed in human odontoma-derived mesenchymal cells (HODC) compared to adult dental mesenchymal stem cells (aDSC), as well as morphological and histological characteristics of odontoma were analyzed.

METHODS

aDSCs were isolated from normal teeth, and HODCs were isolated from surgically removed odontoma mass. Morphological and histological evaluations were performed to compare between compound odontomas and normal premolars. RT-PCR and real-time PCR were performed to identify LHX8 mRNA expression in the HODCs and aDSCs. LHX8 protein expression levels were observed by immunoblotting and immunofluorescent staining.

RESULTS

The compound odontoma was composed of multiple tooth-like structures, which contained disorganized but recognizable enamel matrix, dentin, pulp, and cementum. LHX8 mRNA and LHX8 protein expressions were all higher in HODCs compared to those in aDSCs examined by RT-PCR, immunoblot, and immunofluorescent staining. Especially, real-time PCR showed 2.77-fold higher LHX8 expression in HODCs than in normal periodontal ligament stem cells (PDLSCs), while alveolar bone marrow stem cells (ABMSCs) expressed 0.12-fold LHX8 than PDLSCs.

CONCLUSIONS

Based on these observations, LHX8 might play an important role in odontoma formation. This is the first report regarding the comparison of LHX8 expression between HODC and normal aDSCs and its overexpression in human samples. The specific mechanism of LHX8 in odontoma morphogenesis awaits further study.

Regulation and/or repression of cholinergic differentiation of murine embryonic stem cells using RNAi directed against transcription factor L3/Lhx8

Techniques for controlling the expression of a specific gene in embryonic stem cells are effective and important for clarifying the functions of the gene. Regarding differentiation of cells into nervous system components, these techniques would play key roles in elucidating, not only the differentiation mechanisms of neuronal and glial cells, but also how neuronal phenotypes are determined. In this chapter, we describe an RNA interference method for suppressing cholinergic differentiation in murine embryonic stem cells by knockdown of expression of the transcription factor L3/Lhx8, a Lim homeobox gene family protein. This method will greatly facilitate functional analyses of the factors involved in neuronal differentiation and regeneration and contribute to cell transplantation studies.

LIM homeodomain transcription factor-dependent specification of bipotential MGE progenitors into cholinergic and GABAergic striatal interneurons

Coordination of voluntary motor activity depends on the generation of the appropriate neuronal subtypes in the basal ganglia and their integration into functional neuronal circuits. The largest nucleus of the basal ganglia, the striatum, contains two classes of neurons: the principal population of medium-sized dense spiny neurons (MSNs; 97-98% of all striatal neurons in rodents), which project to the globus pallidus and the substantia nigra, and the locally projecting striatal interneurons (SINs; 2-3% in rodents). SINs are further subdivided into two non-overlapping groups: those producing acetylcholine (cholinergic) and those producing gamma-amino butyric acid (GABAergic). Despite the pivotal role of SINs in integrating the output of striatal circuits and the function of neuronal networks in the ventral forebrain, the lineage relationship of SIN subtypes and the molecular mechanisms that control their differentiation are currently unclear. Using genetic fate mapping, we demonstrate here that the majority of cholinergic and GABAergic SINs are derived from common precursors generated in the medial ganglionic eminence during embryogenesis. These precursors express the LIM homeodomain protein Lhx6 and have characteristics of proto-GABAergic neurons. By combining gene expression analysis with loss-of-function and misexpression experiments, we provide evidence that the differentiation of the common precursor into mature SIN subtypes is regulated by the combinatorial activity of the LIM homeodomain proteins Lhx6, Lhx7 (Lhx8) and Isl1. These studies suggest that a LIM homeodomain transcriptional code confers cell-fate specification and neurotransmitter identity in neuronal subpopulations of the ventral forebrain.

The LIM homeodomain transcription factors Lhx6 and Lhx7 are key regulators of mammalian dentition

Genes encoding LIM homeodomain transcription factors are implicated in cell type specification and differentiation during embryogenesis. Two closely related members of this family, Lhx6 and Lhx7, are expressed in the ectomesenchyme of the maxillary and mandibular processes and have been suggested to control patterning of the first branchial arch (BA1) and odontogenesis. However, mice homozygous for single mutations either have no cranial defects (Lhx6) or show only cleft palate (Lhx7). To reveal the potential redundant activities of Lhx6 and Lhx7 in cranial morphogenesis, we generated mice with all combinations of wild-type and mutant alleles. Double homozygous mice have characteristic defects of the cranial skeleton and die shortly after birth, most likely because of cleft palate. In addition, Lhx6/7 deficient embryos lack molar teeth. The absence of molars in double mutants is not due to patterning defects of BA1 but results from failure of specification of the molar mesenchyme. Despite molar agenesis, Lhx6/7-deficient animals have normal incisors which, in the maxilla, are flanked by a supernumerary pair of incisor-like teeth. Our experiments demonstrate that the redundant activities of the LIM homeodomain proteins Lhx6 and Lhx7 are critical for craniofacial development and patterning of mammalian dentition.

Knockdown of the L3/Lhx8 gene suppresses cholinergic differentiation of murine embryonic stem cell-derived spheres

L3/Lhx8, a member of the Lim-homeobox gene family, is selectively and specifically expressed in the murine embryonic medial ganglionic eminence (MGE). Our previous study demonstrated that L3/Lhx8-deficient mice specifically lack cholinergic neurons in the basal forebrain. In this manuscript, we report the in vitro effects of reduced L3/Lhx8 gene expression on cholinergic differentiation in murine embryonic stem (ES) cell-derived spheres without dissociation. The knockdown of L3/Lhx8 gene expression dramatically decreased the cholinergic phenotype of spheres without altering other known phenotypes (TuJ1, GABA and GFAP). These results strongly suggest that L3/Lhx8 is a key factor for cholinergic differentiation of murine ES cell-derived spheres and is involved in basal forebrain development.

Lhx6 activity is required for the normal migration and specification of cortical interneuron subtypes

The cerebral cortex contains two main neuronal cell populations, the excitatory glutamatergic (pyramidal) neurons and the inhibitory interneurons, which synthesize GABA and constitute 20-30% of all cortical neurons. In contrast to the mostly homogeneous population of projection neurons, cortical interneurons are characterized by remarkable morphological, molecular, and functional diversity. Among the markers that have been used to classify cortical interneurons are the calcium-binding proteins parvalbumin and calretinin and the neuropeptide somatostatin, which in rodents identify mostly nonoverlapping interneuron subpopulations. Pyramidal neurons are born during embryogenesis in the ventricular zone of the dorsal telencephalon, whereas cortical interneurons are generated in the subpallium and reach the cortex by tangential migration. On completion of tangential migration, cortical interneurons switch to a radial mode of migration and enter the cortical plate. Although the mechanisms that control the generation of interneuron diversity are currently unknown, it has been proposed that their site of origin in the ventral forebrain determines their specification into defined neurochemical subgroups. Here, we show that Lhx6, a gene induced in the medial ganglionic eminence and maintained in parvalbumin- and somatostatin-positive interneurons, is required for the specification of these neuronal subtypes in the neocortex and the hippocampus. We also show that Lhx6 activity is required for the normal tangential and radial migration of GABAergic interneurons in the cortex.

L3/Lhx8 is a pivotal factor for cholinergic differentiation of murine embryonic stem cells

L3/Lhx8 is a member of the LIM-homeobox gene family. Previously, we demonstrated that L3/Lhx8-null mice specifically lacked cholinergic neurons in the basal forebrain. In the present study, we conditionally suppressed L3/Lhx8 function during retinoic acid-induced neural differentiation of a murine embryonic stem (ES) cell line using an L3/Lhx8-targeted small interfering RNA (siRNA) produced by an H1.2 promoter-driven vector. Our culture conditions induced efficient differentiation of the ES cells into neurons and astrocytes, but far less efficient differentiation into oligodendrocytes. Suppression of L3/Lhx8 expression by siRNA led to a dramatic decrease in the number of cells positive for the cholinergic marker ChAT, and overexpression of L3/Lhx8 recovered this effect. However, no significant changes were observed in the number of Tuj1+ neurons and GABA+ cells. These results strongly suggest that L3/Lhx8 is a key factor in the cholinergic differentiation of murine ES cells and is involved in basal forebrain development.

Expression profiles of cIRF6, cLHX6 and cLHX7 in the facial primordia suggest specific roles during primary palatogenesis

BACKGROUND

The LIM-homeodomain transcription factors LHX7 and LHX6 have been implicated in palatogenesis in mice and thus may also contribute to the incidence of isolated palatal clefts and/or clefts of the lip and primary palate (CL/P) in humans. Causative mutations in the transcription factor IRF6 have also been identified in two allelic CL/P syndromes and common polymorphisms in the same gene are significantly associated with non-syndromal CL/P in different populations.

RESULTS

Here we report the isolation of chick orthologues of LHX7, LHX6 and IRF6 and the first characterisation of their profiles of expression during morphogenesis of the midface with emphasis on the period around formation of the primary palate. LHX7 and LHX6 expression was restricted to the ectomesenchyme immediately underlying the ectoderm of the maxillary and mandibular primordia as well as to the lateral globular projections of the medial nasal process, again underlying the pre-fusion primary palatal epithelia. In contrast, IRF6 expression was restricted to surface epithelia, with elevated levels around the frontonasal process, the maxillary primordia, and the nasal pits. Elsewhere, high expression was also evident in the egg tooth primordium and in the apical ectodermal ridge of the developing limbs.

CONCLUSION

The restricted expression of both LHX genes and IRF6 in the facial primordia suggests roles for these gene products in promoting directed outgrowth and fusion of the primary palate. The manipulability, minimal cost and susceptibility of chicks to CL/P will enable more detailed investigations into how perturbations of IRF6, LHX6 and LHX7 contribute to common orofacial clefts.

GABAergic specification in the basal forebrain is controlled by the LIM-hd factor Lhx7

We present evidence for a temporal control of GABAergic neurotransmitter specification in the basal forebrain orchestrated by the LIM-homeodomain factor Lhx7. In Xenopus, using in vivo overexpression experiments, we show that x-Lhx7 and x-Nkx2.1 inhibit GABAergic specification in the Dlx-expressing areas of the forebrain (subpallium and diencephalon). In addition, x-Lhx7 almost totally represses GABAergic differentiation at early but not late embryonic stages in subpallial mouse primary neurons in culture, indicating that x-Lhx7 is not able to withdraw the GABAergic phenotype once it is acquired. Moreover, anatomical data show striking correlations between x-Lhx7 expression and the GABAergic/cholinergic phenotypes. These functional and anatomical observations suggest a sequential role for x-Lhx7 in neurotransmitter specification. Thus, x-Lhx7 would first prevent a pool of cells to become GABAergic early in development and then promote cholinergic differentiation later on in this pool. We propose two distinct modulatory roles for a single LIM-hd factor, depending on the developmental time window.

The Arabidopsis zinc finger-homeodomain genes encode proteins with unique biochemical properties that are coordinately expressed during floral development

Arabidopsis (Arabidopsis thaliana) contains approximately 100 homeobox genes, many of which have been shown to play critical roles in various developmental processes. Here we characterize the zinc finger-homeodomain (ZF-HD) subfamily of homeobox genes, consisting of 14 members in Arabidopsis. We demonstrate that the HDs of the ZF-HD proteins share some similarities with other known HDs in Arabidopsis, but they contain distinct features that cluster them as a unique class of plant HD-containing proteins. We have carried out mutational analyses to show that the noncanonical residues present in the HDs of this family of proteins are important for function. Yeast (Saccharomyces cerevisiae) two-hybrid matrix analyses of the ZF-HD proteins reveal that these proteins both homo- and heterodimerize, which may contribute to greater selectivity in DNA binding. These assays also show that most of these proteins do not contain an intrinsic activation domain, suggesting that interactions with other factors are required for transcriptional activation. We also show that the family members are all expressed predominantly or exclusively in floral tissue, indicating a likely regulatory role during floral development. Furthermore, we have identified loss-of-function mutations for six of these genes that individually show no obvious phenotype, supporting the idea that the encoded proteins have common roles in floral development. Based on these results, we propose the ZF-HD gene family encodes a group of transcriptional regulators with unique biochemical activities that play overlapping regulatory roles in Arabidopsis floral development.

LIM-homeodomain genes in mammalian development and human disease

The human and mouse genomes each contain at least 12 genes encoding LIM homeodomain (LIM-HD) transcription factors. These gene regulatory proteins feature two LIM domains in their amino termini and a characteristic DNA binding homeodomain. Studies of mouse models and human patients have established that the LIM-HD factors are critical for the development of specialized cells in multiple tissue types, including the nervous system, skeletal muscle, the heart, the kidneys, and endocrine organs such as the pituitary gland and the pancreas. In this article, we review the roles of the LIM-HD proteins in mammalian development and their involvement in human diseases.

Loss of forebrain cholinergic neurons and impairment in spatial learning and memory in LHX7-deficient mice

The identification of the genetic determinants specifying neuronal networks in the mammalian brain is crucial for the understanding of the molecular and cellular mechanisms that ultimately control cognitive functions. Here we have generated a targeted allele of the LIM-homeodomain-encoding gene Lhx7 by replacing exons 3-5 with a LacZ reporter. In heterozygous animals, which are healthy, fertile and have no apparent cellular deficit in the forebrain, b-galactosidase activity reproduces the pattern of expression of the wild-type Lhx7 locus. However, homozygous mutant mice show severe deficits in forebrain cholinergic neurons (FCNs), while other classes of forebrain neurons appear unaffected. Using the LacZ reporter as a marker, we show that in LHX7-deficient mice FCN progenitors survive but fail to generate cholinergic interneurons in the striatum and cholinergic projection neurons in the basal forebrain. Analysis of behaviour in a series of spatial and non-spatial learning and memory tasks revealed that FCN ablation in Lhx7 mutants is associated with severe deficits in spatial but only mild impairment of non-spatial learning and memory. In addition, we found no deficit in long-term potentiation in mutant animals, suggesting that FCNs modulate hippocampal function independently of its capacity to store information. Overall our experiments demonstrate that Lhx7 expression is required for the specification or differentiation of cholinergic forebrain neurons involved in the processing of spatial information.

L3/Lhx8 is involved in the determination of cholinergic or GABAergic cell fate

The LIM homeobox family of transcription factors is involved in many processes during the development of the mammalian central nerves system. L3, also called Lhx8 (L3/Lhx8), is a recently identified member of the LIM homeobox gene family and is selectively expressed in the medial ganglionic eminence (MGE). Our previous study demonstrated that L3/Lhx8-null mice specifically lacked cholinergic neurons in the basal forebrain. In this study, we reduced L3/Lhx8 function in the murine neuroblastoma cell line, Neuro2a (N2a), using L3/Lhx8-targeted small interfering RNA (siRNA) produced by H1.2 promoter-driven vector. The levels of cholinergic markers per cell were diminished without a reduction in the number of marker-positive cells. Intriguingly, GABAergic marker expression and the number of GABAergic cells were dramatically increased in the differentiating L3/Lhx8-knockdown N2a. These results suggest the possibility that L3/Lhx8 is involved in the determination of transmitter phenotypes (GABAergic or cholinergic cell fate) in a population of neurons during basal forebrain development.

The LIM homeobox gene, L3/Lhx8, is necessary for proper development of basal forebrain cholinergic neurons

Basal forebrain cholinergic neurons (BFCNs) are involved in cognitive functions such as learning and memory, and are affected in several neurodegenerative diseases (e.g. Alzheimer's disease). Despite their importance, the molecular mechanisms of their development are not fully elucidated. A recent report demonstrated that some BFCNs in adult rat are positive for L3/Lhx8, a LIM homeobox transcription factor. To examine the function of L3/Lhx8 in the development of BFCNs, we generated L3/Lhx8 gene-disrupted mice. In these mice, cells expressing cholinergic neuron markers, such as choline acetyltransferase, vesicular acetylcholine transporter and p75 low-affinity NGF receptor, were markedly reduced in the basal forebrain, whereas other cholinergic neurons including brain stem and spinal motor neurons expressed the markers. Neurotransmitter phenotypes other than cholinergic in the basal forebrain appeared intact. From these results, we suggested that L3/Lhx8 has a pivotal and specific role in the development and/or maintenance of BFCNs.

The LIM-homeobox gene Lhx8 is required for the development of many cholinergic neurons in the mouse forebrain

Forebrain cholinergic neurons play important roles as striatal local circuit neurons and basal telencephalic projection neurons. The genetic mechanisms that control development of these neurons suggest that most of them are derived from the basal telencephalon where Lhx8, a LIM-homeobox gene, is expressed. Here we report that mice with a null mutation of Lhx8 are deficient in the development of forebrain cholinergic neurons. Lhx8 mutants lack the nucleus basalis, a major source of the cholinergic input to the cerebral cortex. In addition, the number of cholinergic neurons is reduced in several other areas of the subcortical forebrain in Lhx8 mutants, including the caudate-putamen, medial septal nucleus, nucleus of the diagonal band, and magnocellular preoptic nucleus. Although cholinergic neurons are not formed, initial steps in their specification appear to be preserved, as indicated by a presence of cells expressing a truncated Lhx8 mRNA and mRNA of the homeobox gene Gbx1. These results provide genetic evidence supporting an important role for Lhx8 in development of cholinergic neurons in the forebrain.

Expression and role of Lhx8 in murine tooth development

We examined the expression and possible functions of Lhx8, a member of the LIM-homeobox gene family, during tooth morphogenesis of the mouse. Lhx8 was expressed in the dental mesenchyme between the bud and early bell stage of the molar tooth germ. Tooth germ explants from embryonic day 12.5 mice treated for 5 to 7 days with antisense-oligodeoxynucleotides (AS-ODN) against Lhx8 showed a marked decrease in the number of mesenchymal cells. The explants treated with AS-ODN for 11 to 14 days were filled with a large number of undifferentiated epithelial cells and a limited number of undifferentiated mesenchymal cells, but did not contain a tooth germ. Treatment of explants with AS-ODN for 7 days suppressed the proliferation of dental mesenchymal cells and induced apoptosis; the latter was confirmed by histochemical and ultrastructural examinations. Moreover, the expression of Lhx6, Msx1, Msx2, Bmp4 and Gsc, which are also known to be involved in tooth morphogenesis, were suppressed after the application of AS-ODN against Lhx8 for 7 days. The present results suggest that Lhx8 plays an important role in the survival of mesenchymal cells of the tooth germ during development.

Comparison of the expression patterns of two LIM-homeodomain genes, Lhx6 and L3/Lhx8, in the developing palate

OBJECTIVES

To compare and contrast the gene expression of two LIM-homeobox type transcription factors, Lhx6 and L3/Lhx8, in secondary palate formation.

METHODS

In situ hybridization histochemistry with digoxygenin (DIG) labelled cRNA probes specific for Lhx6 and L3/Lhx8.

MATERIALS

Serial cryo-sections of embryonic day (E)13.5, 14.5, and 15.5 mice (C57BL/6).

OUTCOME MEASURE

Comparison of the signal intensities of NBT/BCIP precipitate by alkaline phosphatase conjugated anti-DIG antibody.

RESULTS

From E13.5 to E15.5, Lhx6 and L3/Lhx8 signals are detected in palatal mesenchyme, but the L3/Lhx8 signal is much more intense than the Lhx6 signal. In palatal epithelium, covering the mesenchyme, Lhx6 mRNA is transiently expressed at E14.5, while L3/Lhx8 mRNA expression is never detected throughout the development.

CONCLUSION

Lhx6 and L3/Lhx8 functions may be partially redundant in the mesenchyme of the secondary palate, but not in the palatal epithelium.

The homeobox genes Lhx7 and Gbx1 are expressed in the basal forebrain cholinergic system

The specific combination of homeobox genes is proposed to be decisive in the terminal differentiation of neuronal systems. In order to identify combined expression of homeobox genes in the ventral forebrain, a reverse transcriptase-polymerase chain reaction strategy using degenerated primers was employed. We identified, amongst others, Lhx7 and Gbx1, displaying a marked overlapping expression in septal and pallidal areas. Gbx1 and Lhx7 were both expressed in those adult brain nuclei that collectively form the basal forebrain cholinergic system, a prime target of neurodegeneration in Alzheimer's disease. Indeed, we detected Lhx7 within cholinergic neurons, whereas the related Lhx6 gene was found in adjacent neurons. From these data we suggest that combined expression of Lhx7 and Gbx1 plays a role in the development of the cholinergic system of the basal forebrain. It is speculated that both genes remain participating in molecular processes in the adult cholinergic neurons, and can be employed to study regulation and survival of these neurons under normal and pathological conditions.

The subventricular zone: source of neuronal precursors for brain repair

The subventricular zone (SVZ) is a major germinal zone which persists in the adult brain. The SVZ contains cells that self renew and continuously produce new neurons and glia. In this chapter we discuss the development, architecture and function of the adult SVZ, as well as the fate of SVZ cells after transplantation. We focus on identification of neural stem cells, factors which regulate neurogenesis and mechanisms for neuronal migration through the adult brain. Detailed understanding of these processes is necessary to utilize the SVZ as a source of neuronal and glial precursors for genetic manipulation, transplantation or brain self repair.

Functions of LIM-homeobox genes

Homeobox genes play fundamental roles in development. They can be subdivided into several subfamilies, one of which is the LIM-homeobox subfamily. The primary structure of LIM-homeobox genes has been remarkably conserved through evolution. Have their functions similarly been conserved? A host of new data has been derived from mutational analysis in diverse organisms, such as nematodes, flies and vertebrates. These studies have revealed a prominent involvement of LIM-homeodomain proteins in tissue patterning and differentiation, and their function in neural patterning is evident in all organisms studied to date. Here, we summarize the recent findings on LIM-homeobox gene function, compare the function of these genes from different organisms and describe specific co-factor requirements.

Isolated cleft palate in mice with a targeted mutation of the LIM homeobox gene lhx8

Formation of the mammalian secondary palate is a highly regulated and complex process whose impairment often results in cleft palate, a common birth defect in both humans and animals. Loss-of-function analysis has linked a growing number of genes to this process. Here we report that Lhx8, a recently identified LIM homeobox gene, is expressed in the mesenchyme of the mouse palatal structures throughout their development. To test the function of Lhx8 in vivo, we generated a mutant mouse with a targeted deletion of the Lhx8 gene. Our analysis of the mutant animals revealed a crucial role for Lhx8 in palatogenesis. In Lhx8 homozygous mutant embryos, the bilateral primordial palatal shelves formed and elevated normally, but they often failed to make contact and to fuse properly, resulting in a cleft secondary palate. Because development of other craniofacial structures appeared normal, the impaired palatal formation in Lhx8-mutant mice was most likely caused by an intrinsic primary defect in the mesenchyme of the palatal shelves. The cleft palate phenotype observed in Lhx8-mutant mice suggests that Lhx8 is a candidate gene for the isolated nonsyndromic form of cleft palate in humans.

Molecular cloning and distinct developmental expression pattern of spliced forms of a novel zinc finger gene wiz in the mouse cerebellum

In the course of a study conducted to identify the mouse homologue of Drosophila eyes absent (eya), we isolated a novel mouse cDNA fragment which show little homology to eya but encodes a protein with Krüppel (C2H2)-type zinc finger motifs. By further screening using this cDNA fragment as a probe, we obtained the short and long forms of full-length cDNAs, which were apparently alternatively spliced products from one gene. Since both mRNAs encode proteins with widely-interspaced zinc finger motifs, we termed this gene wiz and refer to the short and long wiz transcripts as wizS and wizL, respectively. In situ hybridization studies using the probe against the region common to wizS and wizL showed that these mRNAs were expressed abundantly in the granule cell layers of the mouse cerebellum, the olfactory bulb, and the dentate gyrus, whereas the same technique using the probe against only wizL could not detect positive signals in the developing cerebellum, indicating that there is no expression of wizL mRNA there. Northern blot and in situ hybridization analyses demonstrated that the extracerebellar regions expressed both wizS and wizL mRNAs from the midgestational period to adulthood. The finding that two types of wiz transcripts (wizS and wizL) are expressed with different developmental patterns might indicate separate transcription functions in the cerebellar granule cells and the extracerebellar regions.

Structure and chromosomal localization of a murine LIM/homeobox gene, Lhx8

Lhx8 is a LIM-homeodomain protein, containing two tandemly repeated LIM motifs and a hemeodomain. The expression of Lhx8 is limited spatially to the medical ganglionic eminence and the mesenchyme surrounding the oral cavity and temporally from middle embryonic to early postnatal development, suggesting a role for Lhx8 in differentiation of certain neurons and mesenchymal cells, just as the other LIM-homeodomain proteins are implicated in determining the fates of certain cell types. Here we report the structure and the chromosomal localization of the Lhx8 gene. The gene is composed of nine exons and eight introns. The first LIM domain is coded by two exons, exons 2 and 3, and the second by a single exon, exon 4. The homeodomain is encoded by three exons, exons 6, 7, and 8. In situ chromosomal hybridization demonstrated that the Lhx8 gene was localized in the distal region of mouse chromosome 3.

DeLIMiting development

LIM-homeodomain transcription factors (LIM-HD) regulate expression of genes that pattern the body and generate cell specificity during development in invertebrates and vertebrates. It is especially interesting that most are expressed in and participate in the development of the nervous system. LIM-HD proteins are themselves regulated by both intramolecular and intermolecular interactions mediated by the LIM domains. LIM domains positively regulate LIM-HD activity by promoting protein-protein interactions that allow cooperative binding to regulatory regions of tissue-specific promoters. They also negatively regulate LIM-HD activity, possibly by preventing HD association with DNA. Interaction of LIM domains with other proteins relieves this interference, permitting DNA binding and providing a mechanism for refining LIM-HD activity. The recurrence of LIM-HD proteins in fundamental developmental processes emphasizes the importance of their function and regulation and provides an opportunity to identify mechanisms and molecules underlying patterning and cell specification.

Differential expression of LIM-homeodomain genes in the embryonic murine brain

Homeobox-containing genes are associated with the control of various stages in embryogenesis. LIM-homeodomain genes have been implicated in the control of differentiation of specific cell types in both neuronal and non-neuronal tissues, yet there have been few studies comparing the expression domains of these genes in the developing brain of a single species. In the present study, we isolated murine LIM-homeodomain genes (LH-2, lim-1, and L3) by reverse transcription-polymerase chain reaction (RT-PCR) method and investigated their expression domains in the embryonic brain by in situ hybridization analysis. These mRNAs showed mutually exclusive expression patterns suggesting that these factors may be involved in region-specific differentiation in the developing brain.