Gene regulation and differentiation in vertebrate ocular tissues

Diverse Evolutionary Origins and Mechanisms of Lens Regeneration

In this review, we compare and contrast the three different forms of vertebrate lens regeneration: Wolffian lens regeneration, cornea-lens regeneration, and lens regeneration from lens epithelial cells. An examination of the diverse cellular origins of these lenses, their unique phylogenetic distribution, and the underlying molecular mechanisms, suggests that these different forms of lens regeneration evolved independently and utilize neither conserved nor convergent mechanisms to regulate these processes.

A novel CpG island set identifies tissue-specific methylation at developmental gene loci

CpG islands (CGIs) are dense clusters of CpG sequences that punctuate the CpG-deficient human genome and associate with many gene promoters. As CGIs also differ from bulk chromosomal DNA by their frequent lack of cytosine methylation, we devised a CGI enrichment method based on nonmethylated CpG affinity chromatography. The resulting library was sequenced to define a novel human blood CGI set that includes many that are not detected by current algorithms. Approximately half of CGIs were associated with annotated gene transcription start sites, the remainder being intra- or intergenic. Using an array representing over 17,000 CGIs, we established that 6%–8% of CGIs are methylated in genomic DNA of human blood, brain, muscle, and spleen. Inter- and intragenic CGIs are preferentially susceptible to methylation. CGIs showing tissue-specific methylation were overrepresented at numerous genetic loci that are essential for development, including HOX and PAX family members. The findings enable a comprehensive analysis of the roles played by CGI methylation in normal and diseased human tissues.

The human genome contains about 22,000 genes, each encoding one of the proteins required for human life. A particular cell type (e.g., blood, skin, etc.) expresses a specific subset of protein genes and silences the remainder. To shed light on the mechanisms that cause genes to be activated or shut down, we studied DNA sequences called “CpG islands” (CGIs). These sequences are found at over half of all human genes and can exist in either the active or silent state depending on the presence or absence of methyl groups on the DNA. We devised a method for purifying all CGIs and showed that, unexpectedly, only half occur at the beginning of genes near the promoter, the rest occurring within or between genes. Notably, methylation of CGIs causes stable gene silencing. We tested 17,000 CGIs in four human tissues and found that 6%–8% were methylated in each. Genes whose protein products play an essential role during embryonic development were preferentially methylated, suggesting that gene expression during development could be regulated by CGI methylation.

CpG island methylation, an epigenetic phenomenon usually associated with abnormality in disease, is little characterised in the context of "normal" human cells. Here we highlight tissue-specific CpG Island methylation, which frequently associates with developmental genes.

Expression of an Olfactomedin-Related Gene in Rat Hair Follicular Papilla Cells

Follicular papilla (FP) cells, but not their closely related dermal fibroblasts, can maintain hair growth suggesting cell type-specific molecular signals. To define the molecular differences between these two cell types, we generated a subtraction complementary DNA (cDNA) library highly enriched in FP-specific cDNA. Differential screening identified FP-1 as the most abundant cDNA sequence in this subtraction library. FP-1 message RNA is highly abundant in cultured rat vibrissa FP cells, can be detected at very low levels in the stomach and the ovary, and is undetectable in cultured dermal fibroblasts and in 16 rat non-follicular tissues. The full-length, 2.3 kb FP-1 cDNA encodes a protein of 549 amino acids harboring a signal peptide, collagen triple helix repeats, and an olfactomedin-like domain. Monospecific rabbit antibodies to FP-1 recognize in cultured FP cells a single approximately 72 kDa glycoprotein with a approximately 60 kDa protein core. FP-1 protein is expressed in vivo in a hair cycle-dependent manner, as it can be detected in FP during anagen, but not in catagen and telogen phases of the hair cycle. FP-1 is presumably a highly specific extracellular matrix protein synthesized by FP cells and may be involved in the organization of FP during certain phases of normal or pathological hair growth.

Early regeneration genes: Building a molecular profile for shared expression in cornea-lens transdifferentiation and hindlimb regeneration in Xenopus laevis

Recent studies in Xenopus laevis have begun to compare gene expression during regeneration with that of the original development of specific structures (e.g., the hindlimb and lens), while other studies have sought differences in gene expression between regeneration-competent and regeneration-incompetent stages. To determine whether there are any similarities between the regeneration of different structures, we have used a differential screen to seek shared early gene expression between hindlimb regeneration and cornea-lens transdifferentiation in the Xenopus tadpole. We have isolated 13 clones representing genes whose expression is up-regulated within the first few days of both regenerating processes and which are not demonstrably up-regulated in the context of basic wound healing. Furthermore, all of these genes also show prominent late embryonic expression. The expression patterns and putative identities of all 13 genes are presented, and a model is considered that allows us to characterize and profile important changes in gene expression, which might be shared among various regenerating and developmental systems.

The cellular and molecular bases of vertebrate lens regeneration

Lens regeneration takes place in some vertebrates through processes of cellular dedifferentiation and transdifferentiation, processes by which certain differentiated cell types can give rise to others. This review describes the principal forms of lens regeneration that occur in vivo as well as related in vitro systems of transdifferentiation. Classic experimental studies are reviewed that define the tissue interactions that trigger these events in vivo. Recent molecular analyses have begun to identify the genes associated with these processes. These latter studies generally reveal tremendous similarities between embryonic lens development and lens regeneration. Different models are proposed to describe basic molecular pathways that define the processes of lens regeneration and transdifferentiation. Finally, studies are discussed suggesting that fibroblast growth factors play key roles in supporting the process of lens regeneration. Retinoids, such as retinoic acid, may also play important roles in this process.

Adult neural stem cells: plasticity and developmental potential

Stem cells play an essential role during the processes of embryonic tissue formation and development and in the maintenance of tissue integrity and renewal throughout adulthood. The differentiation potential of stem cells in adult tissues has been thought to be limited to cell lineages present in the organ from which they derive, but there is evidence that somatic stem cells may display a broader differentiation repertoire. This has been documented for bone marrow stem cells (which can give rise to muscle, hepatic and brain cells) and for muscle precursors, which can turn into blood cells. The adult central nervous system (CNS) has long been considered incapable of cell renewal and structural remodeling. Recent findings indicate that, even in postnatal and adult mammals, neurogenesis does occur in different brain regions and that these regions actually contain adult stem cells. These cells can be expanded both in vivo and ex vivo by exposure to different combinations of growth factors and subsequently give rise to a differentiated progeny comprising the major cell types of the CNS. Almost paradoxically, adult neural stem cells display a multipotency much broader than expected, since they can differentiate into non-CNS mesodermal-derivatives, such as blood cells and skeletal muscle cells. We review the recent findings documenting this unforeseen plasticity and unexpected developmental potential of somatic stem cells in general and of neural stem cells in particular. To better introduce these concepts, some basic notions on the functional properties of adult neural stem cells will also be discussed, particularly focusing on the emerging role of the microenvironment in determining and maintaining their peculiar characteristics.

Evolutionary relationships of the metazoan beta gamma-crystallins, including that from the marine sponge Geodia cydonium

beta gamma-crystallins are one major component of vertebrate lenses. Here the isolation and characterization of a cDNA, coding for the first beta gamma-crystallin molecule from an invertebrate species, the marine sponge Geodia cydonium, is described. The size of the transcript as determined by Northern blotting was 0.7 kb in length. The deduced amino acid sequence consists of 163 aa residues and comprises four repeated motifs which compose the two domains of the beta gamma-crystallin. Motif 3 contains the characteristic beta gamma-crystallin 'Greek key' motif signature, while in each of the three other repeats, one aa residue is replaced by an aa with the same physico-chemical property. The sponge peptide shows striking similarities to vertebrate beta gamma-crystallins. Analysis by neighbour joining of the sponge motifs with the two motifs present in spherulin 3a of Physarum polycephalum shows that motif 4 of the sponge beta gamma-crystallin was added as the last single sequence to the tree. The data support the view that the beta gamma-crystallin superfamily, present in eukaryotes, evolved from a common ancestor including also the sponge beta gamma-crystallin.

Cellular and molecular features of lens differentiation: a review of recent advances

In this paper, the more recent literature pertaining to differentiation in the developing vertebrate lens is reviewed in relation to previous work. The literature reviewed reveals that the developing lens has been, and will continue to be, a useful model system for the examination of many fundamental processes occurring during embryonic development. Areas of lens development reviewed here include: the induction and early embryology of the lens; lens cell culture techniques; the role of growth factors and cytokines; the involvement of gap junctions in lens cell-cell communication; the role of cell adhesion molecules, integrins, and the extracellular matrix; the role of the cytoskeleton; the processes of programmed cell death (apoptosis) and lens fibre cell denucleation; the involvement of Pax and Homeobox genes; and crystallin gene regulation. Finally, some speculation is provided as to possible directions for further research in lens development.

Ocular retardation mouse caused by Chx10 homeobox null allele: impaired retinal progenitor proliferation and bipolar cell differentiation

Ocular retardation (or) is a murine eye mutation causing microphthalmia, a thin hypocellular retina and optic nerve aplasia. Here we show that mice carrying the OrJ allele have a premature stop codon in the homeobox of the Chx10 gene, a gene expressed at high levels in uncommitted retinal progenitor cells and mature bipolar cells. No CHX10 protein was detectable in the retinal neuroepithelium of orJ homozygotes. The loss of CHX10 leads both to reduced proliferation of retinal progenitors and to a specific absence of differentiated bipolar cells. Other major retinal cell types were present and correctly positioned in the mutant retina, although rod outer segments were short and retinal lamination was incomplete. These results indicate that Chx10 is an essential component in the network of genes required for the development of the mammalian eye, with profound effects on retinal progenitor proliferation and bipolar cell specification or differentiation. off

New perspectives on eye evolution

The highly complex eyes of vertebrates, insects and molluscs have long been considered to be of independent evolutionary origin. Recently, however, Pax-6, a highly conserved transcription factor, has been identified as a key regulator of eye development in both mammals and flies. Homologues of Pax-6 have also been identified in species from other phyla, including molluscs. The wide variety of eyes in the animal kingdom may, therefore, have evolved from a single ancestral photosensitive origin.

Transgenic models for eye malformations

Several lines of transgenic mice developing eye malformations have been described in the literature and appear to be of increasing interest for the study of eye teratology in humans, since gene expression and regulation can be studied in the developing animal. Transgenic applications are briefly described here and an overview of existing transgenic mouse models carrying different eye abnormalities is given according to the major diagnosis (e.g., cataract, microphthalmia, anterior segment dysgenesis, retinal dysplasia). Interestingly, many transgenic models exhibit pathological findings similar to those observed in human pediatric ophthalmology. Unfortunately, detailed embryological studies in transgenic mice bearing congenital eye malformations are not available for all lines. Thus, the importance of creating further transgenic models to study the function of morphogenes and growth factors in eye development is also discussed.