Expression of Pax-6 mRNA in the retinal degeneration (rd) mouse

RNA-binding protein Rbpms is represented in human retinas by isoforms A and C and its transcriptional regulation involves Sp1-binding site

Rbpms (RNA-binding protein with multiple splicing) is a member of the RRM (RNA Recognition Motif) family of RNA-binding proteins, which is expressed as multiple alternatively spliced transcripts encoding different protein isoforms. We have shown earlier that Rbpms expression in the retina is restricted to retinal ganglion cells (RGCs), and have characterized this gene as a marker for RGCs. The aim of this study was to identify isoforms representing Rbpms in human retinas and to analyze its transcriptional regulation. We found that Rbpms is expressed as transcription variants 1 and 3 encoding isoforms A and C, respectively. These isoforms are encoded by the same first 6 exons but have different C-terminal ends encoded by exon 8 in variant 1 and exon 7 in variant 3. Computational analysis of the Rbpms 5' untranslated and flanking regions reveals the presence of three CpG islands and four predicted promoter regions (PPRs). The effect of PPR 1 (- 1672/- 1420) and PPR2 (- 330/- 79) on transcriptional activation was minimal, whereas PPR 3 (- 73/+ 177) and PPR4 (+ 274/+ 524) induced the expression by ~ 7 and ninefold compared to control, respectively. The maximum activity, a 30-fold increase above the control level, was obtained from the construct containing both PPRs 3 and 4. Site-directed mutagenesis of several cis-elements within PPR3 and PPR4 including five for Sp1, one for AP1, and two for NF-kB showed that mutation of the first three and especially the first GC box resulted in a threefold downregulation of gene expression. AP1, NF-kB, and two downstream Sp1 sites had no significant effect on expression level. The possible involvement of the GC box 1 at position - 54 in transcriptional regulation of Rbpms was corroborated by EMSA, which showed formation of a DNA-protein complex in the presence of the oligonucleotide corresponding to this Sp1-binding site.

Heat shock proteins in the retina: Focus on HSP70 and alpha crystallins in ganglion cell survival

Heat shock proteins (HSPs) belong to a superfamily of stress proteins that are critical constituents of a complex defense mechanism that enhances cell survival under adverse environmental conditions. Cell protective roles of HSPs are related to their chaperone functions, antiapoptotic and antinecrotic effects. HSPs' anti-apoptotic and cytoprotective characteristics, their ability to protect cells from a variety of stressful stimuli, and the possibility of their pharmacological induction in cells under pathological stress make these proteins an attractive therapeutic target for various neurodegenerative diseases; these include Alzheimer's, Parkinson's, Huntington's, prion disease, and others. This review discusses the possible roles of HSPs, particularly HSP70 and small HSPs (alpha A and alpha B crystallins) in enhancing the survival of retinal ganglion cells (RGCs) in optic neuropathies such as glaucoma, which is characterized by progressive loss of vision caused by degeneration of RGCs and their axons in the optic nerve. Studies in animal models of RGC degeneration induced by ocular hypertension, optic nerve crush and axotomy show that upregulation of HSP70 expression by hyperthermia, zinc, geranyl-geranyl acetone, 17-AAG (a HSP90 inhibitor), or through transfection of retinal cells with AAV2-HSP70 effectively supports the survival of injured RGCs. RGCs survival was also stimulated by overexpression of alpha A and alpha B crystallins. These findings provide support for translating the HSP70- and alpha crystallin-based cell survival strategy into therapy to protect and rescue injured RGCs from degeneration associated with glaucomatous and other optic neuropathies.

PAX6 Expression and Retinal Cell Death in a Transgenic Mouse Model for Acute Angle-Closure Glaucoma

PURPOSE

PAX6 is a highly conserved protein essential for the control of eye development both in invertebrates and vertebrates. PAX6 expression persists in the adult inner retina, but little is known about its functions after completion of retinal differentiation. Therefore, we investigated PAX6 expression in wild-type and calcitonin receptor-like receptor transgenic (CLR(SMαA)) mice with angle-closure glaucoma.

METHODS

Intraocular pressure was measured by indentation tonometry in anesthetized mice. Eyes of mice of both genotypes were enucleated at various ages and retinas were processed for morphological analysis and PAX6 immunostaining. The content of PAX6 in retinal extracts was estimated by Western blot analysis. Retinal expression of glaucoma-related genes was analyzed by reverse transcription-polymerase chain reaction.

RESULTS

Control mice showed normal retinal morphology between p22 and p428 with steady PAX6 expression in the ganglion cell layer (GCL) and the inner nuclear layer (INL). CLR(SMαA) mice examined between p22 and p82 exhibited increased intraocular pressure and a progressive decrease in cell number including PAX6-expressing cells in the GCL. The INL was not affected up to postnatal day 42. Later, a significant increase in PAX6-expressing cells concomitant with an overall loss of cells was observed in the INL of CLR(SMαA) as compared with control mice. Retinal up-regulation of glaucoma-related genes was furthermore observed.

CONCLUSIONS

Distinctive changes of PAX6 expression in the inner retina of CLR(SMαA) mice suggest a role in regulatory mechanisms involved in glaucoma-related retinal cell death. The selective increase of PAX6 expression in the degenerating INL of CLR(SMαA) mice may represent an attempt to preserve retinal cytoarchitecture.

Crystallins in retinal ganglion cell survival and regeneration

Crystallins are heterogeneous proteins classified into alpha, beta, and gamma families. Although crystallins were first identified as the major structural components of the ocular lens with a principal function to maintain lens transparency, further studies have demonstrated the expression of these proteins in a wide variety of tissues and cell types. Alpha crystallins (alpha A and alpha B) share significant homology with small heat shock proteins and have chaperone-like properties, including the ability to bind and prevent the precipitation of denatured proteins and to increase cellular resistance to stress-induced apoptosis. Stress-induced upregulation of crystallin expression is a commonly observed phenomenon and viewed as a cellular response mechanism against environmental and metabolic insults. However, several studies reported downregulation of crystallin gene expression in various models of glaucomatous nerodegeneration suggesting that that the decreased levels of crystallins may affect the survival properties of retinal ganglion cells (RGCs) and thus, be associated with their degeneration. This hypothesis was corroborated by increased survival of axotomized RGCs in retinas overexpressing alpha A or alpha B crystallins. In addition to RGC protective functions of alpha crystallins, beta and gamma crystallins were implicated in RGC axonal regeneration. These findings demonstrate the importance of crystallin genes in RGC survival and regeneration and further in-depth studies are necessary to better understand the mechanisms underlying the functions of these proteins in healthy RGCs as well as during glaucomatous neurodegeneration, which in turn could help in designing new therapeutic strategies to preserve or regenerate these cells.

Developmental and daily expression of the Pax4 and Pax6 homeobox genes in the rat retina: localization of Pax4 in photoreceptor cells

Pax4 is a homeobox gene encoding Pax4, a transcription factor that is essential for embryonic development of the endocrine pancreas. In the pancreas, Pax4 counters the effects of the related transcription factor, Pax6, which is known to be essential for eye morphogenesis. In this study, we have discovered that Pax4 is strongly expressed in retinal photoreceptors of the rat. Pax4 expression is not detectable in the foetal eye; however, postnatal Pax4 transcript levels rapidly increase. In contrast, Pax6 exhibits an inverse developmental pattern of expression being more strongly expressed in the foetal eye. Histological analysis revealed that Pax4 mRNA is exclusively expressed in the retinal photoreceptors, whereas Pax6 mRNA and protein are present in the inner nuclear layer and in the ganglion cell layer of the mature retina. In the adult retina, Pax4 transcripts exhibit a diurnal rhythm with maximal levels occurring during the light period, whereas retinal Pax6 transcript levels do not change throughout the day. The daily changes in Pax4 expression may contribute to daily changes in function in the differentiated retinal photoreceptor.

Modulation of alpha and beta crystallin expression in rat retinas with ocular hypertension-induced ganglion cell degeneration

The expression of alpha (alphaA and alphaB) and beta (betaA1/A3, betaA2, betaA4, and betaB2) crystallin genes were analyzed at the mRNA and protein levels in rat retinas with ocular hypertension-induced ganglion cell death. An animal model with progressive loss of retinal ganglion cells (RGC) was generated by elevation of intraocular pressure (IOP). The estimated RGC loss was approximately 8% and 20% at 2 and 5 weeks post IOP elevation, respectively. mRNA and protein quantification showed that alpha and beta crystallin genes were downregulated at both transcriptional (alphaA, alphaB, betaA1/A3, betaA4, and betaB2 approximately 50% and betaA2~40%) and protein (alphaA~50%, alphaB~63%, betaA1/A3~70%, and betaB2~38%) levels 2 weeks after IOP elevation. In experimental retinas 5 weeks after IOP elevation, the levels of crystallin mRNAs were higher than at 2 weeks and were comparable to that of control retinas. However, the levels of the corresponding proteins were still lower (alphaA, alphaB, and betaB2 approximately 37% and betaA1/A3~70%) than in control retinas. Furthermore, we found that the expression of these genes in the retina is predominantly localized to the cells in the GCL and to a lesser degree in the INL and ONL. Colocalization of the crystallin-positive and Fluorogold retrogradely labeled cells indicated that the cells expressing alpha and beta crystallins in the GCL are RGCs. In summary, we showed that alpha and beta crystallins are expressed in the retina predominantly by RGCs and that their expression is affected by ocular hypertension.

Requirement for Bhlhb5 in the specification of amacrine and cone bipolar subtypes in mouse retina

The mammalian retina comprises six major neuronal cell types and one glial type that are further classified into multiple subtypes based on their anatomical and functional differences. Nevertheless, how these subtypes arise remains largely unknown at the molecular level. Here, we demonstrate that the expression of Bhlhb5, a bHLH transcription factor of the Olig family, is tightly associated with the generation of selective GABAergic amacrine and Type 2 OFF-cone bipolar subtypes throughout retinogenesis. Targeted deletion of Bhlhb5 results in a significant reduction in the generation of these selective bipolar and amacrine subtypes. Furthermore, although a Bhlhb5-null mutation has no effect on the expression of bHLH-class retinogenic genes, Bhlhb5 expression overlaps with that of the pan-amacrine factor NeuroD and the expression of Bhlhb5 and NeuroD is negatively regulated by ganglion cell-competence factor Math5. Our results reveal that a bHLH transcription factor cascade is involved in regulating retinal cell differentiation and imply that Bhlhb5 functions downstream of retinogenic factors to specify bipolar and amacrine subtypes.

Neuronal differentiation of bone marrow-derived stromal stem cells involves suppression of discordant phenotypes through gene silencing

Tissue engineering involves the construction of transplantable tissues in which bone marrow aspirates may serve as an accessible source of autogenous multipotential mesenchymal stem cells. Increasing reports indicate that the lineage restriction of adult mesenchymal stem cells may be less established than previously believed, and stem cell-based therapeutics await the establishment of an efficient protocol capable of achieving a prescribed phenotype differentiation. We have investigated how adult mouse bone marrow-derived stromal cells (BMSCs) are guided to neurogenic and osteogenic phenotypes. Naïve BMSCs were found surprisingly active in expression of a wide range of mRNAs and proteins, including those normally reported in terminally differentiated neuronal cells and osteoblasts. The naïve BMSCs were found to exhibit voltage-dependent membrane currents similar to the neuronally guided BMSCs, although with smaller amplitudes. Once BMSCs were exposed to the osteogenic culture condition, the neuronal characteristics quickly disappeared. Our data suggest that the loss of discordant phenotypes during BMSC differentiation cannot be explained by the selection and elimination of unfit cells from the whole BMSC population. The percent ratio of live to dead BMSCs examined did not change during the first 8-10 days in either neurogenic or osteogenic differentiation media, and cell detachment was estimated at <1%. However, during this period, bone-associated extracellular matrix genes were selectively down-regulated in neuronally guided BMSCs. These data indicate that the suppression of discordant phenotypes of differentiating adult stem cells is achieved, at least in part, by silencing of superfluous gene clusters.

Neuronal differentiation of stem cells isolated from adult muscle

Lineage uncommitted pluripotent stem cells reside in the connective tissue of skeletal muscle. The present study was carried out with pluripotent stem cells (PPSCs) isolated from 6-month old rat muscle. Before differentiation, these cells were vimentin+, CD90+, CD45-, and varied in their expression of CD34. The PPSCs were expanded as non-adherent aggregates under similar conditions to those used to generate neurospheres from embryonic or neural stem cells. The PPSC-derived neurospheres were positive for nestin, an early marker present in neuronal precursors, and expressed the two alternative mRNA forms of the neuroectodermal marker Pax-6, as well as mRNA for Oct-4, a gene related to the pluripotentiality of stem cells. To confirm their neural potential, PPSC-derived neurospheres were plated on coated coverslips under varying conditions: Neurobasal medium with N2 or B27, and either NT3 or BDNF. After 4-6 days the cells expressed neuronal (Tuj1+, NF68), astrocytic (GFAP) and oligodendrocytic (MOSP+, MBP+) markers, both by immunocytochemistry and RT-PCR. In addition, PPSCs were cultured as monolayers under adherent conditions, exposed to growth factors and defined differentiating conditions for 5 hr, and subsequently kept for 2 days in a maturation medium. At this point they gave rise to a mixed population of early neural progenitors (Nestin+ or NG2+), immature and mature neurons (Tuj1+ and NF145+) and myelin producing oligodendrocytes (CNPase + and MOSP+). Our study shows that PPSCs present in adult muscle can overcome germ lineage restrictions and express the molecular characteristics of brain cells. Therefore, PPSCs isolated from adult muscle could provide a novel source for autologous cell replacement in neurodegenerative and demyelinating diseases.

Math3 and NeuroD regulate amacrine cell fate specification in the retina

The basic helix-loop-helix genes Math3 and NeuroD are expressed by differentiating amacrine cells, retinal interneurons. Previous studies have demonstrated that a normal number of amacrine cells is generated in mice lacking either Math3 or NEUROD: We have found that, in Math3-NeuroD double-mutant retina, amacrine cells are completely missing, while ganglion and Müller glial cells are increased in number. In the double-mutant retina, the cells that would normally differentiate into amacrine cells did not die but adopted the ganglion and glial cell fates. Misexpression studies using the developing retinal explant cultures showed that, although Math3 and NeuroD alone only promoted rod genesis, they significantly increased the population of amacrine cells when the homeobox gene Pax6 or Six3 was co-expressed. These results indicate that Math3 and NeuroD are essential, but not sufficient, for amacrine cell genesis, and that co-expression of the basic helix-loop-helix and homeobox genes is required for specification of the correct neuronal subtype.