An avian alpha B-crystallin. Non-lens expression and sequence similarities with both small (HSP27) and large (HSP70) heat shock proteins

Characterization of the antigenic phenotype of αB-crystallin-expressing peripapillary glial cells in the developing chick retina

Radial glia are transdifferentiated into astrocytes within the developing brain and spinal cord. The neural retina contains Müller cells, which are retinal radial glia. Some of the cells that surround the optic nerve head among Müller cells in the chicken retina are called peripapillary glial cells (PPGCs). PPGCs express different molecules compared to typical Müller cells. However, an antigenic PPGC phenotype has not yet been clearly established. In this study, we classified the antigenic PPGC phenotypes and identified the differentiation stages of these cells. At embryonic day (E)8, αB-crystallin-positive PPGCs had a bipolar shape with long processes that traversed entire layers of the retina. Pax2 and vimentin were expressed in αB-crystallin-positive PPGCs. Glial fibrillary acidic protein (GFAP) immunoreactivity was not observed in PPGCs. At E18, αB-crystallin immunoreactivity disappeared from the vitread processes of PPGCs. However, the PPGC cell bodies and ventricular processes contained αB-crystallin protein, and the PPGCs retained the same Pax2-positive/vimentin-positive/GFAP-negative profile as that seen at E8. At post-hatch day 120, αB-crystallin and Pax2 immunoreactivity was not observed, but vimentin and GFAP expression was clearly observed in the presumptive location of the PPGCs. Furthermore, these two proteins overlapped within that location. Considering that vimentin expression is prolonged until the post-hatching period in chicken brain, these findings suggest that Pax2-negative/vimentin-positive/GFAP-positive PPGCs are phenotypically identical to mature astrocytes in this avian species.

Genome-wide analysis and expression profiling of the small heat shock proteins in zebrafish

Small Heat Shock Proteins (sHSPs) have important roles in preventing disease and promoting resistance to environmental stressors. Mutations in any one of a number of sHSPs, including HSP27 (HSPB1), HSP22 (HSPB8), alphaA-crystallin (HSPB4), or alphaB-crystallin (HSPB5) can result in neuronal degeneration, myopathy, and/or cataract in humans. Ten sHSPs are known in humans, and thirteen have been identified in teleost fish. Here we report the identification of thirteen zebrafish sHSPs. Using a combination of phylogenetic analysis and analysis of synteny, we have determined that ten are likely orthologs of human sHSPs. We have used quantitative RT-PCR to determine the relative expression levels of all thirteen sHSPs during development and in response to heat shock. Our findings indicate that most of the zebrafish sHSPs are expressed during development, and five of these genes are transcriptionally upregulated by heat shock at one or more stages of development.

Changes in albumin precursor and heat shock protein 70 expression and their potential role in response to corneal epithelial wound repair

Many proteins displayed differential expression (either up- or down-regulation) when proteome of migrating and non-migrating epithelium was assessed using 2-DE and ESI-Q-TOF MS/MS. From the up-regulated set, we have identified for the first time a 69-kDa albumin precursor protein with four peptides sequences and 70-kDa heat shock protein (hsp70) with one peptide in the active phase of cell migration (48 h) during the healing process. Western blot analysis was used to further characterize these proteins at different phases (24, 48 and 72 h) of healing. An increase in the mRNA expression (measured using RT-PCR) in the active migration phase (48 h) for albumin precursor and hsp70 was also observed. Furthermore, co-immunoprecipitation studies with anti-albumin precursor and anti-hsp70 antibodies, followed by immunoblotting with anti-fibronectin antibody demonstrated a novel and biologically relevant interaction between albumin precursor protein and fibronectin in corneal epithelial wound healing but not with hsp70. The increased gene and protein expression of albumin and hsp70 during the active phase of cell migration (48 h) in the corneal epithelium suggests their possible role in corneal wound healing. These findings may have broader implications for developing therapeutic strategies for treating wound healing disorders.

Association of clathrin, AP-2 adaptor and actin cytoskeleton with developing interlocking membrane domains of lens fibre cells

Interlocking membrane domains are specialized membrane interdigitations in the form of ball-and-sockets and protrusions between lens fibre cells of all species. They are believed to play a key role in maintaining fibre-fibre stability and are therefore, important for normal lens function. Here we report the specific association of the clathrin/AP-2 adaptor complex and the branching F-actin network with the development of interlocking domains in rats and several other species. By thin-section electron microscopy we consistently observed a layer of distinct coating (approximately 25-nm thick) on the concave membrane surface of small and intermediate-sized developing interlocking domains. These membrane coats remarkably resembled the clathrin-coat of endocytic vesicles in which clathrin and the AP-2 adaptor are involved in the induction of coated pit formation during receptor-mediated endocytosis. We hypothesize that the clathrin/AP-2 complex is directly involved in the induction of interlocking domains in fibre cells. By immunoconfocal microscopy, co-labelling of a dotted-pattern of clathrin and AP-2 adaptor antibodies was seen along the cortical fibre cells. Immunoblot analysis further confirmed that clathrin and AP-2 adaptor antibodies specifically stained a polypeptide band of 180 and 106kD, respectively, in the membrane fractions prepared separately from the outer and inner cortical fibres where interlocking domains are abundant but endocytic vesicles are absent. Immunoelectron microscopy showed that the clathrin antibody was localized along the interlocking membrane. In addition, branching actin filament networks were frequently observed within the cytoplasmic compartment of developing interlocking domains by TEM, in consistent with the findings by fluorescence and immunogold labelling of the F-actin antibody in the domains. These results demonstrate for the first time that the clathrin/AP-2 complex plays a new role for the formation of interlocking domains in lens fibre cells. Branching actin networks and possibly other cytoskeletal components are also associated with the development and maintenance of these interlocking domains. The coordinated 'pulling and pushing' actions generated by the clathrin/AP-2 complex and branching actin networks during interlocking domain formation are discussed.

Heat shock proteins of chicken lens

The presence of heat shock proteins HSP-40, HSP-70, and HSc-70 in adult and embryonic chicken lenses were determined. The epithelium, cortex, and nucleus of adult chicken lens were separated and tested for the presence of heat shock proteins (hsps) by western blot, using specific antibodies for HSP-40, HSP-70, and HSc-70. Water soluble (WSF) and water insoluble fractions (WIF) of embryonic chicken lenses were isolated and tested for the presence of HSP-40, HSP-70, and HSc-70 by immunoblot. Embryonic chicken lens sections were also analyzed for the presence of heat shock proteins by immunofluorescence technique. Data obtained from these experiments revealed that HSP-40, HSP-70, and HSc-70 are present in all areas of both adult and embryonic chicken lens. Presence of hsps protein in the deep cortex and nucleus is intriguing as no detectable metabolic activities are reported in this area. However it can be proposed that hsps HSP-40, HSP-70, and HSc-70 can interact with protein of these areas and protect them from stress induced denaturation.

In situ immunohistochemical study of Bcl-2 and heat shock proteins in human corneal endothelial cells during corneal storage

AIM

To investigate the expression of Bcl-2 and heat shock proteins (HSPs), which are known to increase cell survival, in human corneal endothelial cells (HCECs) of corneas stored in organ culture.

METHODS

32 paired corneas were randomly assigned to either a short or a long storage time. The flat mounts of endothelium were examined after immunostaining with monoclonal antibodies to Bcl-2 and HSP 27, 60, 70, and 90.

RESULTS

HCECs expressed generally all the proteins studied. Bcl-2 expression was weaker in the long stored corneas (p=0.035). There was no relation between immunostaining, age, sex, or death to culture time. Frequently some Descemet membranes carried negative cells preferentially located in folds and exhibiting morphological changes consistent with swelling cells corresponding to early stages of apoptosis.

CONCLUSION

Expression of these cytoprotective proteins reflects the high level of HCEC resistance to stresses induced by organ culture. The decreased immunostaining of Bcl-2 in the long storage group could act in cellular loss currently observed with storage time. The negativity of Bcl-2 and HSP labelling in corneal folding may be related to apoptosis.

Dissociation of human alphaB-crystallin aggregates by thiocyanate is structurally and functionally reversible

Conformational modifications and changes in the aggregation state of human alphaB-crystallin were investigated at different concentrations of SDS, KBr, urea, and NH4SCN and at different temperatures. Intrinsic fluorescence measurements indicated complete and reversible unfolding of the protein at 2 M NH4SCN, whereas the concentration of urea required for complete and irreversible unfolding was 6 M. Gel permeation chromatography indicated almost complete dissociation of the micelle-like aggregate of alphaB-crystallin in 2 M NH4SCN, but only partial dissociation into large-sized aggregates in 6 M urea. Thiocyanate-treated alphaB-crystallin recovered its chaperone-like activity upon dilution of the dissociating agent, whereas the urea-treated protein did not.

Site-directed mutations within the core "alpha-crystallin" domain of the small heat-shock protein, human alphaB-crystallin, decrease molecular chaperone functions

Site-directed mutagenesis was used to evaluate the effects on structure and function of selected substitutions within and N-terminal to the core "alpha-crystallin" domain of the small heat-shock protein (sHsp) and molecular chaperone, human alphaB-crystallin. Five alphaB-crystallin mutants containing single amino acid substitutions within the core alpha-crystallin domain displayed a modest decrease in chaperone activity in aggregation assays in vitro and in protecting cell viability of E. coli at 50 degrees C in vivo. In contrast, seven alphaB-crystallin mutants containing substitutions N-terminal to the core alpha-crystallin domain generally resembled wild-type alphaB-crystallin in chaperone activity in vitro and in vivo. Size-exclusion chromatography, ultraviolet circular dichroism spectroscopy and limited proteolysis were used to evaluate potential structural changes in the 12 alphaB-crystallin mutants. The secondary, tertiary and quaternary structures of mutants within and N-terminal to the core alpha-crystallin domain were similar to wild-type alphaB-crystallin. SDS-PAGE patterns of chymotryptic digestion were also similar in the mutant and wild-type proteins, indicating that the mutations did not introduce structural modifications that altered the exposure of proteolytic cleavage sites in alphaB-crystallin. On the basis of the similarities between the sequences of human alphaB-crystallin and the sHsp Mj HSP16.5, the only sHsp for which there exists high resolution structural information, a three-dimensional model for alphaB-crystallin was constructed. The mutations at sites within the core alpha-crystallin domain of alphaB-crystallin identify regions that may be important for the molecular chaperone functions of sHsps.

Phosphorylations of alpha A- and alpha B-crystallin

In addition to being refractive proteins in the vertebrate lens, the two alpha-crystallin polypeptides (alpha A and alpha B) are also molecular chaperones that can protect proteins from thermal aggregation. The alpha B-crystallin polypeptide, a functional member of the small heat shock family, is expressed in many tissues in a developmentally regulated fashion, is stress-inducible, and is overexpressed in many degenerative diseases and some tumors indicating that it plays multiple roles. One possible clue to alpha-crystallin functions is the fact that both polypeptides are phosphorylated on serine residues by cAMP-dependent and cAMP-independent mechanisms. The cAMP-independent pathway is an autophosphorylation that has been demonstrated in vitro, depends on magnesium and requires cleavage of ATP. Disaggregation of alpha A-, but not alpha B-crystallin into tetramers results in an appreciable increase in autophosphorylation activity, reminiscent of other heat shock proteins, and suggests the possibility that changes in the aggregation state of alpha A-crystallin are involved in yet undiscovered signal transduction pathways. The alpha-crystallin polypeptides differ with respect to their abilities to undergo cAMP-dependent phosphorylation, with preference given to the alpha B-crystallin chain. These differences and complexities in alpha-crystallin phosphorylations, coupled with the differences in expression patterns of the two alpha-crystallin polypeptides, are consistent with the idea that each polypeptide has distinctive structural and metabolic roles.

alpha-Crystallin sequences support a galliform/anseriform clade

An unresolved issue in higher avian systematics is the position of the fowl-like and the duck-like birds, Galliformes and Anseriformes, respectively. Most studies place these orders at the base of the neognath radiation. While DNA hybridization data support a sister-group relationship of Galliformes and Anseriformes, macromolecular sequence analyses have not yet been able to provide a clear-cut answer. In this study, we present nucleotide sequences coding for the eye lens proteins alpha A- and alpha B-crystallin of a palaeognath, a galliform, an anseriform, and two other neognathous birds. Phylogenetic analyses of this data set clearly support a galliform/anseriform clade, to the exclusion of other neognaths.

Alternative transcriptional initiation and alternative use of polyadenylation signals in the alphaB-crystallin gene expressed in different chicken tissues

Overexpression of alphaB-crystallin is associated with numerous neurodegenerative diseases and abnormal cell growth patterns. To study the mechanisms involved in the control of the transcriptional activity of the gene we have characterized its expression in different chicken tissues. The sequence of the alphaB-crystallin cDNA isolated from chicken testis and 6-day-old chick embryo is highly homologous to the duck alphaB-crystallin cDNA and differs from the previously reported chicken lens alphaB-crystallin cDNA in the 5' untranslated region (5'-UTR) and in one amino acid of the coding sequence. Four forms of the alphaB-crystallin cDNA detected in chicken testes arise from the use of alternative transcription initiation sites and alternative polyadenylation signals. The two principal hybridizing bands found in lens and embryonic tissues possess a short 5'-UTR and differ in the length of the 3'-UTR. Forms with longer 5'-UTR are present in testis, muscle, and heart. The use of different start sites and polyadenylation signals could modulate transcriptional activity and the stability of the messages. The expression of the alphaB-crystallin gene decreases from day 6 to day 8 of chick embryogenesis, in parallel with the expression of the polyubiquitin gene UbII.

Human alphaB-crystallin. Small heat shock protein and molecular chaperone

The polymerase chain reaction was used to amplify a cDNA sequence encoding the human alphaB-crystallin. The amplified cDNA fragment was cloned into the bacterial expression vector pMAL-c2 and expressed as a soluble fusion protein coupled to maltose-binding protein (MBP). After maltose affinity chromatography and cleavage from MBP by Factor Xa, the recombinant human alphaB-crystallin was separated from MBP and Factor Xa by anion exchange chromatography. Recombinant alphaB-crystallin was characterized by SDS-polyacrylamide electrophoresis (PAGE), Western immunoblot analysis, Edman degradation, circular dichroism spectroscopy, and size exclusion chromatography. The purified crystallin migrated on SDS-PAGE to an apparent molecular weight (Mr approximately 22,000) that corresponded to total native human alpha-crystallin and was recognized on Western immunoblots by antiserum raised against human alphaB-crystallin purified from lens homogenates. Chemical sequencing, circular dichroism spectroscopy, and size exclusion chromatography demonstrated that the recombinant crystallin had properties similar or identical to its native counterpart. Both recombinant alphaB-crystallin and MBP-alphaB fusion protein associated to form high molecular weight complexes that displayed chaperone-like function by inhibiting the aggregation of alcohol dehydrogenase at 37 degrees C and demonstrated the importance of the C-terminal domain of alphaB-crystallin for chaperone-like activity.

Modulation of the chaperone-like activity of bovine alpha-crystallin

The effects of pantethine, glutathione, and selected chemical reagents on the anti-aggregation activity of alpha-crystallin was evaluated. Protein aggregation was monitored by light scattering of solutions of denatured beta L-crystallin or alcohol dehydrogenase (ADH). The ratios of beta L-crystallin/alpha-crystallin and ADH/alpha-crystallin were adjusted so that partial inhibition of protein aggregation at 60 degrees C or 37 degrees C, respectively, was observed and modulation of the chaperone action of alpha-crystallin could be evaluated easily with selected endogenous metabolites. Enhancement of the anti-aggregation activity in the beta L-crystallin assay was strongest with pantethine, which appeared to interact with alpha-crystallin. Enhancement of the anti-aggregation activity in the ADH assay was strongest with glutathione which appeared to interact with ADH. The results indicated that the products of common metabolic pathways can modulate the chaperone-like effects of alpha-crystallin on protein aggregation.

Protein sequences indicate that turtles branched off from the amniote tree after mammals

The phylogenetic relationships among the major groups of amniote vertebrates remain a matter of controversy. Various alternatives for the position of the turtles have been proposed, branching off either before or after the mammals. To discover the phylogenetic position of turtles in relation to mammals and birds, we have determined cDNA sequences for the eye lens proteins alpha A- and alpha B-crystallin of the red-eared slider turtle (Trachemys scripta elegans). In addition, databases were searched for turtle protein sequences, for which mammalian, avian, and outgroup orthologs were available. All sequences were analyzed by three phylogenetic tree reconstruction methods (neighbor-joining, maximum parsimony, and maximum likelihood). Including the alpha-crystallins, 7 out of 12 proteins support a sister-group relation of turtles and birds with all 3 methods. For each of the other five proteins no topology was consistently preferred by the three approaches. Analyses of the combined amino acid data (1,695 aligned sites) also give extremely strong evidence that turtles are nearer to birds, indicating that mammals branched off before the divergence between turtles and birds occurred.

The duck gene for alpha B-crystallin shows evolutionary conservation of discrete promoter elements but lacks heat and osmotic shock response

The gene for alpha Beta-crystallin from a bird (the domestic duck, Anas platyrhynchos) has been cloned and sequenced to allow comparison with its mammalian homologues. The duck gene has the same general structure as those of humans and rodents although, unlike those of mammals, the duck gene has two polyadenylation signals at the 3' end. The most interesting comparisons are in the 5'flanking promoter regions. In contrast to the broad conservation of promoter sequence among mammals, only two significant blocks and a few smaller elements have been conserved during evolution in the more distantly related avian gene. Block 1 (-350/-308) corresponds to alpha BE-2, a functional element defined in the mouse gene. Further downstream, block 2 (-98/-65) shows 27/33 identity among all three species but does not correspond to any previously defined element. Other regions are less well-conserved. In particular, putative heat-shock response elements of the mammalian alpha B-crystallin genes are absent from the duck gene. In contrast to the heat and osmotic stress-inducibility of mouse alpha B-crystallin in NIH 3T3 cells, duck alpha B-crystallin showed no inducibility in duck cells in culture. Thus, although high expression in lens is common to alpha B-crystallin genes in birds and mammals, other modes of expression appear to be taxon-specific.

The expanding small heat-shock protein family, and structure predictions of the conserved "alpha-crystallin domain"

The ever-increasing number of proteins identified as belonging to the family of small heat-shock proteins (shsps) and alpha-crystallins enables us to reassess the phylogeny of this ubiquitous protein family. While the prokaryotic and fungal representatives are not properly resolved, most of the plant and animal shsps and related proteins are clearly grouped in distinct clades, reflecting a history of repeated gene duplications. The members of the shsp family are characterized by the presence of a conserved homologous "alpha-crystallin domain," which sometimes is present in duplicate. Predictions are made of secondary structure and solvent accessibility of this domain, which together with hydropathy profiles and intron positions support the presence of two similar hydrophobic beta-sheet-rich motifs, connected by a hydrophilic alpha-helical region. Together with an overview of the newly characterized members of the shsp family, these data help to define this family as being involved as stable structural proteins and as molecular chaperones during normal development and induced under pathological and stressful conditions.

Structure and modifications of the junior chaperone alpha-crystallin. From lens transparency to molecular pathology

alpha-Crystallin is a high-molecular-mass protein that for many decades was thought to be one of the rare real organ-specific proteins. This protein exists as an aggregate of about 800 kDa, but its composition is simple. Only two closely related subunits termed alpha A- and alpha B-crystallin, with molecular masses of approximately 20 kDa, form the building blocks of the aggregate. The idea of organ-specificity had to be abandoned when it was discovered that alpha-crystallin occurs in a great variety of nonlenticular tissues, notably heart, kidney, striated muscle and several tumors. Moreover alpha B-crystallin is a major component of ubiquinated inclusion bodies in human degenerative diseases. An earlier excitement arose when it was found that alpha B-crystallin, due to its very similar structural and functional properties, belongs to the heat-shock protein family. Eventually the chaperone nature of alpha-crystallin could be demonstrated unequivocally. All these unexpected findings make alpha-crystallin a subject of great interest far beyond the lens research field. A survey of structural data about alpha-crystallin is presented here. Since alpha-crystallin has resisted crystallization, only theoretical models of its three-dimensional structure are available. Due to its long life in the eye lens, alpha-crystallin is one of the best studied proteins with respect to post-translational modifications, including age-induced alterations. Because of its similarities with the small heat-shock proteins, the findings about alpha-crystallin are illuminative for the latter proteins as well. This review deals with: structural aspects, post-translational modifications (including deamidation, racemization, phosphorylation, acetylation, glycation, age-dependent truncation), the occurrence outside of the eye lens, the heat-shock relation and the chaperone activity of alpha-crystallin.

Alpha-crystallin/small heat shock protein has autokinase activity

The alpha-crystallins (alpha A and alpha B) are major water-soluble proteins of the transparent eye lens that are expressed in a variety of tissues and can function as molecular chaperones. alpha B-crystallin is also a small heat shock protein associated with numerous degenerative diseases and abnormal growth patterns. Previous experiments have shown that alpha A-and alpha B-crystallin are phosphorylated on specific serine residues by a cAMP-dependent pathway. Here we provide evidence that either total bovine alpha-crystallin or its isolated polypeptides can autophosphorylate serine by a cAMP-independent mechanism in the presence of Mg2+ and [gamma-32P]ATP; the autophosphorylated products isoelectrically focus with the authentic phosphorylated forms of the alpha-crystallin polypeptides. Thus, the alpha A- and alpha B-crystallin/small heat shock protein polypeptides are enzyme-crystallins which may be involved in metabolic pathways important for the development, maintenance, or pathology of the lens and other tissues.