1148. Relationship Between the Duration of Symptoms Before and COVID-19 Disease Outcome After Monoclonal Antibody Therapy

Background

Clinical trials of monoclonal antibodies therapy (MAB) for COVID-19 demonstrated the risk reduction of COVID related hospitalization and death of any cause if administered within the first 7 days from the symptom onset. The Food and Drug Administration (FDA) issued an emergency use authorization (EUA) for MAB within 10 days from the symptom onset. Our objective was to evaluate how duration of symptoms before MAB affects disease outcome following therapy. Figure 1

A Breakthrough in Understanding the Pathogenesis of Molar Hypomineralisation: The Mineralisation-Poisoning Model

Popularly known as "chalky teeth", molar hypomineralisation (MH) affects over 1-in-5 children worldwide, triggering massive amounts of suffering from toothache and rapid decay. MH stems from childhood illness and so offers a medical-prevention avenue for improving oral and paediatric health. With a cross-sector translational research and education network (The D3 Group; thed3group.org) now highlighting this global health opportunity, aetiological understanding is urgently needed to enable better awareness, management and eventual prevention of MH. Causation and pathogenesis of "chalky enamel spots" (i.e., demarcated opacities, the defining pathology of MH) remain unclear despite 100 years of investigation. However, recent biochemical studies provided a pathomechanistic breakthrough by explaining several hallmarks of chalky opacities for the first time. This article outlines these findings in context of previous understanding and provides a working model for future investigations. The proposed pathomechanism, termed "mineralisation poisoning", involves localised exposure of immature enamel to serum albumin. Albumin binds to enamel-mineral crystals and blocks their growth, leading to chalky opacities with distinct borders. Being centred on extracellular fluid rather than enamel-forming cells as held by dogma, this localising pathomechanism invokes a new type of connection with childhood illness. These advances open a novel direction for research into pathogenesis and causation of MH, and offer prospects for better clinical management. Future research will require wide-ranging inputs that ideally should be coordinated through a worldwide translational network. We hope this breakthrough will ultimately lead to medical prevention of MH, prompting global health benefits including major reductions in childhood tooth decay.

100 Years of Chalky Teeth Research: From Pioneering Histopathology to Social Good

One hundred years ago, histopathology pioneer Bernhard Gottlieb described developmentally disrupted teeth as having “chalky enamel” and “chalky spots” that “crumble” easily. He also asked pivotal questions about the pathogenesis of “enamel hypoplasia” that remained enigmatic for almost a century. Today, breakthrough pathomechanistic investigations of chalky enamel are revealing surprising answers, and an allied translational initiative—The D3 Group for developmental dental defects (“D3s”) —is converting such scientific knowledge into social good surrounding prevention of tooth decay. Molar hypomineralisation (MH) affects 1-in-5 children worldwide and is well-evidenced, but poorly recognised, as a principal risk factor for childhood tooth decay. Given MH is causally linked to infantile illness, an exciting corollary is that medical prevention would lead to substantial reductions in decay. Here we reflect on the past century of chalky teeth research and retrace the path leading to recognition of MH as a global health concern. Five research eras, today's four major D3s, and diverse experimental attacks are outlined alongside translational wins that have benefitted global health. Addressing hopes for medical prevention of MH, this centennial year's pathomechanistic discovery is contextualised against past accomplishments and new opportunities. Finally, we note the translational value of accessible infographics for guiding future work, and forecast exciting prospects for the next century.

Pathogenesis of Molar Hypomineralisation: Aged Albumin Demarcates Chalky Regions of Hypomineralised Enamel

Molar hypomineralisation (MH) is becoming globally recognised as a significant public health problem linked to childhood tooth decay. However, with causation and pathogenesis unclear after 100 years of investigation, better pathological understanding is needed if MH is to become preventable. Our studies have implicated serum albumin in an extracellular pathomechanism for chalky enamel, opposing longheld dogma about systemic injury to enamel-forming cells. Hypothesising that chalky enamel arises through developmental exposure to serum albumin, this study used biochemical approaches to characterise demarcated opacities from 6-year molars. Addressing contradictory literature, normal enamel was found to completely lack albumin subject to removal of surface contamination. Querying surface permeability, intact opacities were found to lack salivary amylase, indicating that “enamel albumin” had become entrapped before tooth eruption. Thirdly, comparative profiling of chalky and hard-white enamel supported a dose-response relationship between albumin and clinical hardness of opacities. Moreover, albumin abundance delineated chalky enamel from white transitional enamel at opacity borders. Finally, addressing the corollary that enamel albumin had been entrapped for several years, clear signs of molecular ageing (oxidative aggregation and fragmentation) were identified. By establishing aged albumin as a biomarker for chalky enamel, these findings hold methodological, clinical, and aetiological significance. Foremost, direct inhibition of enamel-crystal growth by albumin (here termed “mineralisation poisoning”) at last provides a cogent explanation for the clinical presentation of demarcated opacities. Together, these findings justify pursuit of an extracellular paradigm for the pathogenesis of MH and offer exciting new prospects for alleviating childhood tooth decay through medical prevention of MH.

Pathogenesis of Molar Hypomineralisation: Hypomineralised 6-Year Molars Contain Traces of Fetal Serum Albumin

Molar Hypomineralisation (MH) is gaining cross-sector attention as a global health problem, making deeper enquiry into its prevention a research priority. However, causation and pathogenesis of MH remain unclear despite 100 years of investigation into “chalky” dental enamel. Contradicting aetiological dogma involving disrupted enamel-forming cells (ameloblasts), our earlier biochemical analysis of chalky enamel opacities implicated extracellular serum albumin in enamel hypomineralisation. This study sought evidence that the albumin found in chalky enamel reflected causal events during enamel development rather than later association with pre-existing enamel porosity. Hypothesising that blood-derived albumin infiltrates immature enamel and directly blocks its hardening, we developed a “molecular timestamping” method that quantifies the adult and fetal isoforms of serum albumin ratiometrically. Applying this novel approach to 6-year molars, both isoforms of albumin were detectable in 6 of 8 chalky opacities examined (corresponding to 4 of 5 cases), indicating developmental acquisition during early infancy. Addressing protein survival, in vitro analysis showed that, like adult albumin, the fetal isoform (alpha-fetoprotein) bound hydroxyapatite avidly and was resistant to kallikrein-4, the pivotal protease involved in enamel hardening. These results shift primary attention from ameloblast injury and indicate instead that an extracellular mechanism involving localised exposure of immature enamel to serum albumin constitutes the crux of MH pathogenesis. Together, our pathomechanistic findings plus the biomarker approach for onset timing open a new direction for aetiological investigations into the medical prevention of MH.

Evidence That Calcium Entry Into Calcium-Transporting Dental Enamel Cells Is Regulated by Cholecystokinin, Acetylcholine and ATP

Dental enamel is formed by specialized epithelial cells which handle large quantities of Ca²⁺ while producing the most highly mineralized tissue. However, the mechanisms used by enamel cells to handle bulk Ca²⁺ safely remain unclear. Our previous work contradicted the dogma that Ca²⁺ is ferried through the cytosol of Ca²⁺-transporting cells and instead suggested an organelle-based route across enamel cells. This new paradigm involves endoplasmic reticulum (ER)-associated Ca²⁺ stores and their concomitant refilling by store-operated Ca²⁺ entry (SOCE) mediated by Ca²⁺ release activated Ca²⁺ (CRAC) channels. Given that Ca²⁺ handling is maximal during the enamel-mineralization stage (maturation), we anticipated that SOCE would also be elevated then. Confirmation was obtained here using single-cell recordings of cytosolic Ca²⁺ concentration ([Ca²⁺]_cyt) in rat ameloblasts. A candidate SOCE agonist, cholecystokinin (CCK), was found to be upregulated during maturation, with Cck transcript abundance reaching 30% of that in brain. CCK-receptor transcripts were also detected and Ca²⁺ imaging showed that CCK stimulation increased [Ca²⁺]_cyt in a dose-responsive manner that was sensitive to CRAC-channel inhibitors. Similar effects were observed with two other SOCE activators, acetylcholine and ATP, whose receptors were also found in enamel cells. These results provide the first evidence of a potential regulatory system for SOCE in enamel cells and so strengthen the Ca²⁺ transcytosis paradigm for ER-based transport of bulk Ca²⁺. Our findings also implicate enamel cells as a new physiological target of CCK and raise the possibility of an auto/paracrine system for regulating Ca²⁺ transport.

Direct evidence that KLK4 is a hydroxyapatite-binding protein

The protease kallikrein 4 (KLK4) plays a pivotal role during dental enamel formation by degrading the major enamel protein, amelogenin, prior to the final steps of enamel hardening. KLK4 dysfunction is known to cause some types of developmental defect in enamel but the mechanisms responsible for transient retention of KLK4 in semi-hardened enamel matrix remain unclear. To address contradictory reports about the affinity of KLK4 for enamel hydroxyapatite-like mineral, we used pure components in quasi-physiological conditions and found that KLK4 binds hydroxyapatite directly. Hypothesising KLK4 self-destructs once amelogenin is degraded, biochemical analyses revealed that KLK4 progressively lost activity, became aggregated, and autofragmented when incubated without substrate in both the presence and absence of reducer. However, with non-ionic detergent present as proxy substrate, KLK4 remained active and intact throughout. These findings prompt a new mechanistic model and line of enquiry into the role of KLK4 in enamel hardening and malformation.

Molar Hypomineralisation: A Call to Arms for Enamel Researchers

Developmental dental defects (DDDs, hereafter “D3s”) hold significance for scientists and practitioners from both medicine and dentistry. Although, attention has classically dwelt on three other D3s (amelogenesis imperfecta, dental fluorosis, and enamel hypoplasia), dental interest has recently swung toward Molar Hypomineralisation (MH), a prevalent condition characterised by well-delineated (“demarcated”) opacities in enamel. MH imposes a significant burden on global health and has potential to become medically preventable, being linked to infantile illness. Yet even in medico-dental research communities there is only narrow awareness of this childhood problem and its link to tooth decay, and of allied research opportunities. Major knowledge gaps exist at population, case and tooth levels and salient information from enamel researchers has sometimes been omitted from clinically-oriented conclusions. From our perspective, a cross-sector translational approach is required to address these complex inadequacies effectively, with the ultimate aim of prevention. Drawing on experience with a translational research network spanning Australia and New Zealand (The D3 Group; www.thed3group.org), we firstly depict MH as a silent public health problem that is generally more concerning than the three classical D3s. Second, we argue that diverse research inputs are needed to undertake a multi-faceted attack on this problem, and outline demarcated opacities as the central research target. Third, we suggest that, given past victories studying other dental conditions, enamel researchers stand to make crucial contributions to the understanding and prevention of MH. Finally, to focus geographically diverse research interests onto this nascent field, further internationalisation of The D3 Group is warranted.

Proteomic Analysis of Dental Tissue Microsamples

Improved understanding of dental enamel development will benefit not only dentistry but also biomedicine more generally. Rat and mouse models of enamel development are relatively well characterized and experimentally powerful. However, the diminutive size of murine teeth makes them difficult to study using standard proteomics approaches. Here, we describe gel-based proteomic methods that enable parallel quantification, identification, and functional characterization of proteins from developing rat and mouse teeth. These refined methods are applicable to other scarce samples including human enamel defects.

A prominent role of PDIA6 in processing of misfolded proinsulin

Despite its critical role in maintaining glucose homeostasis, surprisingly little is known about proinsulin folding in the endoplasmic reticulum. In this study we aimed to understand the chaperones involved in the maturation and degradation of proinsulin. We generated pancreatic beta cell lines expressing FLAG-tagged proinsulin. Several chaperones (including BiP, PDIA6, calnexin, calreticulin, GRP170, Erdj3 and ribophorin II) co-immunoprecipitated with proinsulin suggesting a role for these proteins in folding. To investigate the chaperones responsible for targeting misfolded proinsulin for degradation, we also created a beta cell line expressing FLAG-tagged proinsulin carrying the Akita mutation (Cys96Tyr). All chaperones found to be associated with wild type proinsulin also co-immunoprecipitated with Akita proinsulin. However, one additional protein, namely P58(IPK), specifically precipitated with Akita proinsulin and approximately ten fold more PDIA6, but not other PDI family members, was bound to Akita proinsulin. The latter suggests that PDIA6 may act as a key reductase and target misfolded proinsulin to the ER-degradation pathway. The preferential association of PDIA6 to Akita proinsulin was also confirmed in another beta cell line (βTC-6). Furthermore, for the first time, a physiologically relevant substrate for PDIA6 has been evidenced. Thus, this study has identified several chaperones/foldases that associated with wild type proinsulin and has also provided a comprehensive interactome for Akita misfolded proinsulin.

Pancreatic beta cells are highly susceptible to oxidative and ER stresses during the development of diabetes

The complex interplay of many cell types and the temporal heterogeneity of pancreatic islet composition obscure the direct role of resident alpha and beta cells in the development of Type 1 diabetes. Therefore, in addition to studying islets isolated from non-obese diabetic mice, we analyzed homogeneous cell populations of murine alpha (αTC-1) and beta (NIT-1) cell lines to understand the role and differential survival of these two predominant islet cell populations. A total of 56 proteins in NIT-1 cells and 50 in αTC-1 cells were differentially expressed when exposed to proinflammatory cytokines. The major difference in the protein expression between cytokine-treated NIT-1 and αTC-1 cells was free radical scavenging enzymes. A similar observation was made in cytokine-treated whole islets, where a comprehensive analysis of subcellular fractions revealed that 438 unique proteins were differentially expressed under inflammatory conditions. Our data indicate that beta cells are relatively susceptible to ER and oxidative stress and reveal key pathways that are dysregulated in beta cells during cytokine exposure. Additionally, in the islets, inflammation also leads to enhanced antigen presentation, which completes a three-way insult on beta cells, rendering them targets of infiltrating T lymphocytes.

New paradigms on the transport functions of maturation-stage ameloblasts

Fully matured dental enamel is an architecturally and mechanically complex hydroxyapatite-based bioceramic devoid of most of the organic material that was essential in its making. Enamel formation is a staged process principally involving secretory and maturation stages, each associated with major changes in gene expression and cellular function. Cellular activities that define the maturation stage of amelogenesis include ion (e.g., calcium and phosphate) transport and storage, control of intracellular and extracellular pH (e.g., bicarbonate and hydrogen ion movements), and endocytosis. Recent studies on rodent amelogenesis have identified a multitude of gene products that appear to be linked to these cellular activities. This review describes the main cellular activities of these genes during the maturation stage of amelogenesis.

Gene-expression analysis of early- and late-maturation-stage rat enamel organ

Enamel maturation is a dynamic process that involves high rates of mineral acquisition, associated fluctuations in extracellular pH, and resorption of extracellular enamel proteins. During maturation, ameloblasts change from having a tall, thin, and highly polarized organization, characteristic of the secretory stage, to having a low columnar and widened morphology in the maturation stage. To identify potential differences in gene expression throughout maturation, we obtained enamel organ epithelial cells derived from the early- and late-maturation stages of rat incisor and analyzed the global gene-expression profiles at each stage. Sixty-three candidate genes were identified as having potential roles in the maturation process. Quantitative PCR was used to confirm the results of this genome-wide analysis in a subset of genes. Transcripts enriched during late maturation (n = 38) included those associated with lysosomal activity, solute carrier transport, and calcium signaling. Also up-regulated were transcripts involved in cellular responses to oxidative stress, proton transport, cell death, and the immune system. Transcripts down-regulated during the late maturation stage (n =25) included those with functions related to cell adhesion, cell signaling, and T-cell activation. These results indicate that ameloblasts undergo widespread molecular changes during the maturation stage of amelogenesis and hence provide a basis for future functional investigations into the mechanistic basis of enamel mineralization.

Exclusion of all three calbindins from a calcium-ferry role in rat enamel cells

It is widely accepted that healthy enamel formation depends on a steady supply of calcium, yet only fragmentary understanding exists about the mechanisms underlying transepithelial calcium transport. Several lines of evidence indicate that calcium principally follows a transcellular route, which classically is thought to be facilitated by cytosolic calcium-binding proteins termed calbindins. In enamel cells, however, this 'calcium-ferry' dogma appears to fail as we previously found that the major calbindin in murine enamel cells (calbindin-28 kDa) was down-regulated during the peak period of calcium transport and enamel was formed normally in mice lacking calbindin-28 kDa. It remains to be clarified whether the two other known calbindins could function as calcium ferries instead. This study used biochemical and proteomic approaches to obtain definitive identification and quantification of the 30-kDa calbindin (calretinin) and calbindin-9 kDa (S100-G) in enamel epithelium from rat. By establishing that both of these calbindins contribute insufficient calcium capacities in molars and incisors, our results render the calcium-ferry dogma untenable. Of significance to enamel defects and dental bioengineering, these findings support other evidence for an alternative organelle-based mode of calcium transport (calcium transcytosis) and also implicate S100-G/calbindin-9 kDa, but not calretinin, in a calcium-signaling role during enamel maturation.

Identification of novel candidate genes involved in mineralization of dental enamel by genome-wide transcript profiling

The gene repertoire regulating vertebrate biomineralization is poorly understood. Dental enamel, the most highly mineralized tissue in mammals, differs from other calcifying systems in that the formative cells (ameloblasts) lack remodeling activity and largely degrade and resorb the initial extracellular matrix. Enamel mineralization requires that ameloblasts undergo a profound functional switch from matrix-secreting to maturational (calcium transport, protein resorption) roles as mineralization progresses. During the maturation stage, extracellular pH decreases markedly, placing high demands on ameloblasts to regulate acidic environments present around the growing hydroxyapatite crystals. To identify the genetic events driving enamel mineralization, we conducted genome-wide transcript profiling of the developing enamel organ from rat incisors and highlight over 300 genes differentially expressed during maturation. Using multiple bioinformatics analyses, we identified groups of maturation-associated genes whose functions are linked to key mineralization processes including pH regulation, calcium handling, and matrix turnover. Subsequent qPCR and Western blot analyses revealed that a number of solute carrier (SLC) gene family members were up-regulated during maturation, including the novel protein Slc24a4 involved in calcium handling as well as other proteins of similar function (Stim1). By providing the first global overview of the cellular machinery required for enamel maturation, this study provide a strong foundation for improving basic understanding of biomineralization and its practical applications in healthcare.

ERp29 regulates DeltaF508 and wild-type cystic fibrosis transmembrane conductance regulator (CFTR) trafficking to the plasma membrane in cystic fibrosis (CF) and non-CF epithelial cells

Sodium 4-phenylbutyrate (4PBA) improves the intracellular trafficking of ΔF508-CFTR in cystic fibrosis (CF) epithelial cells. The underlying mechanism is uncertain, but 4PBA modulates the expression of some cytosolic molecular chaperones. To identify other 4PBA-regulated proteins that might regulate ΔF508-CFTR trafficking, we performed a differential display RT-PCR screen on IB3-1 CF bronchiolar epithelial cells exposed to 4PBA. One transcript up-regulated by 4PBA encoded ERp29, a luminal resident of the endoplasmic reticulum (ER) thought to be a novel molecular chaperone. We tested the hypothesis that ERp29 is a 4PBA-regulated ER chaperone that influences ΔF508-CFTR trafficking. ERp29 mRNA and protein expression was significantly increased (∼1.5-fold) in 4PBA-treated IB3-1 cells. In Xenopus oocytes, ERp29 overexpression increased the functional expression of both wild-type and ΔF508-CFTR over 3-fold and increased wild-type cystic fibrosis transmembrane conductance regulator (CFTR) plasma membrane expression. In CFBE41o- WT-CFTR cells, expression of and short circuit currents mediated by CFTR decreased upon depletion of ERp29 as did maturation of newly synthesized CFTR. In IB3-1 cells, ΔF508-CFTR co-immunoprecipitated with endogenous ERp29, and overexpression of ERp29 led to increased ΔF508-CFTR expression at the plasma membrane. These data suggest that ERp29 is a 4PBA-regulated ER chaperone that regulates WT-CFTR biogenesis and can promote ΔF508-CFTR trafficking in CF epithelial cells.

Proteomic analysis of dental tissue microsamples

Improved understanding of dental enamel development will benefit not only dentistry but also biomedicine more generally. Rat and mouse models of enamel development are relatively well characterized and experimentally powerful. However, the diminutive size of murine teeth makes them difficult to study using standard proteomic approaches. Here we describe gel-based proteomic methods that enable parallel quantification, identification, and functional characterization of proteins from developing rat and mouse teeth. These refined methods are also likely to be applicable to other scarce samples.

ERp29 restricts Connexin43 oligomerization in the endoplasmic reticulum

Connexin43 (Cx43) is a gap junction protein that forms multimeric channels that enable intercellular communication through the direct transfer of signals and metabolites. Although most multimeric protein complexes form in the endoplasmic reticulum (ER), Cx43 seems to exit from the ER as monomers and subsequently oligomerizes in the Golgi complex. This suggests that one or more protein chaperones inhibit premature Cx43 oligomerization in the ER. Here, we provide evidence that an ER-localized, 29-kDa thioredoxin-family protein (ERp29) regulates Cx43 trafficking and function. Interfering with ERp29 function destabilized monomeric Cx43 oligomerization in the ER, caused increased Cx43 accumulation in the Golgi apparatus, reduced transport of Cx43 to the plasma membrane, and inhibited gap junctional communication. ERp29 also formed a specific complex with monomeric Cx43. Together, this supports a new role for ERp29 as a chaperone that helps stabilize monomeric Cx43 to enable oligomerization to occur in the Golgi apparatus.

Triplex profiling of functionally distinct chaperones (ERp29/PDI/BiP) reveals marked heterogeneity of the endoplasmic reticulum proteome in cancer

The biomedical need for streamlined approaches to monitor proteome dynamics is growing rapidly. This study examined the ability of a knowledge-based triplex-profiling strategy (i.e., three functionally distinct chaperones, ERp29/PDI/BiP) to clarify uncertainties about how cancer affects the endoplasmic reticulum (ER) proteome. Investigating a wide range of samples at the tissue and cellular levels (>114 samples from 9 tissues of origin), we obtained consistent evidence that the ER proteome undergoes a major but variable expansion in cancer. Three factors having a strong influence on the ER proteome were identified (cancer-cell type, growth rate, culture mode), and the functionally enigmatic chaperone ERp29 was linked distinctively to histogenetic aspects of tumorigenesis. These findings justify pursuit of the ER-proteome as a medical target in cancer, validate ERp29/PDI/BiP profiling as a streamlined yet powerful measure of ER-proteome dynamics, and suggest that biomarker sets based on distinct functionalities could have broader biomedical utility.

Towards second-generation proteome analysis of murine enamel-forming cells

Proteome analysis of rat enamel-forming cells, initiated over a decade ago, has provided valuable insights to enamel biology. In preparation for a more comprehensive, second-generation proteomic exploration, we evaluated an updated microsample-profiling strategy that comprises sequential extraction of enamel epithelium, parallel one- and two-dimensional gel electrophoresis, and mass spectrometric sequence analysis. The results indicated that several hundred proteins, representing various cellular compartments (including membranes), are amenable to identification with a starting tissue volume of <10 microl. With its increased proteomic depth and breadth, this straightforward approach constitutes a major advance from the first-generation work (10-fold increased proteome coverage), although care was needed to ensure a comparably high stringency of protein identification. Expression proteomics has an exciting potential to elucidate the inner workings of murine enamel epithelial cells, leading to an improved understanding of enamel in health and disease.

Proteomic profiling of facial development in chick embryos

Craniofacial disorders are associated with one-third of human birth defects but the underlying molecular and cellular causes remain poorly understood. Proteomics seems well-placed to benefit this medically important area but the scarcity of embryonic tissues poses a major challenge. In this study, we applied a microsample proteomics strategy to investigate the first branchial arch, an embryonic structure crucial for facial development, and found that proteome analysis is both practicable and informative despite the scarcity of tissue. Exploiting the embryonic chick as a tractable source of accurately staged tissue, we developed a sequential extraction procedure to interface with one-dimensional polyacrylamide gel electrophoresis (1-D PAGE) and 2-D PAGE. In 2-D gels, about 8% of the visible proteome changed between embryonic days 3 and 5, and the identities determined for 21 proteins accorded with the rapid growth during this period. These results led to the first molecular identification of chicken alpha-fetoprotein, and an unusual localisation of vimentin to endoderm. With over 470 protein spots accessible, this comparative proteomics approach has good prospects for providing new markers, functional hypotheses and genes to target in functional tests. A broader value of extending these approaches to facial development in other species and to other areas in embryology can be anticipated.

Purification and biochemical characterization of native ERp29 from rat liver

ERp29 is a recently characterized resident of the ER (endoplasmic reticulum) lumen that has broad biological significance, being expressed ubiquitously and abundantly in animal cells. As an apparent housekeeper, ERp29 is thought to be a general folding assistant for secretory proteins and to probably function as a PDI (protein disulphide isomerase)-like molecular chaperone. In the present paper, we report the first purification to homogeneity and direct functional analysis of native ERp29, which has led to the unexpected finding that ERp29 lacks PDI-like folding activities. ERp29 was purified 4800-fold in non-denaturing conditions exploiting an unusual affinity for heparin. Two additional biochemical hallmarks that will assist the classification of ERp29 homologues were identified, namely the idiosyncratic behaviours of ERp29 on size-exclusion chromatography (M(r)<globular homodimer) and SDS/PAGE (M(r)>monomeric mass). In contrast with PDI and parallel-purified co-residents (calreticulin, ERp60), native ERp29 lacked classical chaperone, disulphide reductase and isomerase, and calcium-binding activities. In the chaperone assays, ERp29 neither protected substrate proteins against thermal aggregation nor interacted stably with chemically denatured proteins as detected by cross-linking. ERp29 also did not exhibit helper activity toward calreticulin (chaperone) or PDI and ERp60 (disulphide reductase). By refuting long-standing predictions about chaperone activity, these results expose ERp29 as a functionally distinct member of the ER machinery and prompt a revised hypothesis that ERp29 acts as a non-classical folding assistant. The native preparation and biochemical hallmarks established here provide a useful foundation for ongoing efforts to resolve the functional orphan status of ERp29.

Calbindin Independence of Calcium Transport in Developing Teeth Contradicts the Calcium Ferry Dogma

Cytosolic calcium-binding proteins termed calbindins are widely regarded as a key component of the machinery used to transport calcium safely across cells. Acting as mobile buffers, calbindins are thought to ferry calcium in bulk and simultaneously protect against its potentially cytotoxic effects. Here, we contradict this dogma by showing that teeth and bones were produced normally in null mutant mice lacking calbindin(28kDa). Structural analysis of dental enamel, the development of which depends critically on active calcium transport, showed that mineralization was unaffected in calbindin(28kDa)-null mutants. An unchanged rate of calcium transport was verified by measurements of (45)Ca incorporation into developing teeth in vivo. In enamel-forming cells, the absence of calbindin(28kDa) was not compensated by other cytosolic calcium-binding proteins as detectable by (45)Ca overlay, two-dimensional gel, and equilibrium binding analyses. Despite a 33% decrease in cytosolic buffer capacity, cytotoxicity was not evident in either the null mutant enamel or its formative cells. This is the first definitive evidence that calbindins are not required for active calcium transport, either as ferries or as facilitative buffers. Moreover, in challenging the broader notion of a cytosolic route for calcium, the findings support an alternative paradigm involving passage via calcium-tolerant organelles.

Biophysical characterization of ERp29. Evidence for a key structural role of cysteine 125

ERp29 is a major resident of the endoplasmic reticulum (ER) that seemingly plays an important role in most animal cells. Although a protein-folding association is widely supported, ERp29's specific molecular function remains unknown. A chaperone activity was postulated from evidence that ERp29 forms multimers like the classical ER chaperones, but conflicting results have emerged from our recent studies. Here a biophysical approach was used to clarify this issue and also reveal a key structural role for ERp29's characteristic cysteine, Cys-125. Applying hydrodynamic parameters derived from sedimentation and dynamic light-scattering analyses, a model of ERp29's quaternary structure was assembled from existing tertiary substructures. Comparison with Windbeutel, an ERp29-like protein from fruit fly with specialized chaperone activity, revealed similar tri-lobar gross structures but some finer differences consistent with functional divergence. Solubility and hydrophobic probe assays revealed moderate surface hydrophobicity, which was reduced in mutant ERp29 in which serine replaced Cys-125. This mutant was also relatively labile to proteolytic degradation, providing two reasons for the strict conservation of Cys-125. No multimerization was observed with untagged ERp29, which existed as tight homodimers (K(d) < 50 nm), whereas His-tagged ERp29 artifactually formed 670-kDa oligomers. These findings distinguish ERp29 biophysically from its peers in the ER including Windbeutel, endorsing our postulate that ERp29 adds a distinct type of folding activity to the ER machinery. By invoking novel functional associations for Cys-125 and the adjoining linker, new clues about how ERp29 might work have also arisen.

ERp29, a general endoplasmic reticulum marker, is highly expressed throughout the brain

ERp29 is a recently discovered resident of the endoplasmic reticulum (ER) that is abundant in brain and most other mammalian tissues. Investigations of nonneural secretory tissues have implicated ERp29 in a major role producing export proteins, but a molecular activity remains wanting for this functional orphan. Intriguingly, ERp29 appears to be heavily utilized in the cerebellum, a brain region not conventionally regarded as neurosecretory. To elucidate this functional quandary, we used immunochemical approaches to characterize the regional, cellular, and subcellular distributions of ERp29 in rat brain. Immunohistochemistry revealed ubiquitous expression in neuronal and nonneuronal cells, with a distinctive variation in somatic ERp29 levels. Highly expressing cells were found in diverse locations, implying that ERp29 is not biased towards the cerebellum functionally. Using immunolocalization data mined from the literature, a proteomic profile was developed to assess the functional significance of ERp29's characteristic expression pattern. Surprisingly, ERp29 correlated poorly with classical markers of neurosecretion, but strongly with a variety of major membrane proteins. Together with immunogold localization of ERp29 to somatic ER, these observations led to a novel hypothesis that ERp29 is involved primarily in production of endomembrane proteins rather than proteins destined for export. This study establishes ERp29 as a general ER marker for brain cells and provides a stimulating clue about ERp29's enigmatic function. ERp29 appears to have broad significance for neural pathophysiology, given its ubiquitous distribution and prominence in brain over classical ER residents like BiP and protein disulfide isomerase.

Functional proteomics: The goalposts are moving

Holistic understanding of protein function is a primary goal of the post-genome sequencing era. Functional genomic approaches are powerful and relatively straightforward but produce an incomplete picture at the protein level. Proteomics offers physiologically enriched insights to protein function, and ongoing advances are enabling proteome analyses to proceed with increased depth and efficiency. Exciting discoveries have emerged recently amidst growing awareness of the power of proteomics. However, while proven as a potent discovery tool, proteomics is under pressure to provide improved functional value particularly in concert with other investigative approaches. As reviewed here for ERp29, a recently discovered endoplasmic reticulum protein, the role of novel proteins can remain elusive even after substantial information has accrued. Thousands more proteins of uncertain function will be unveiled in the near future. Consequently, the goalposts are moving for proteomics both through increasing demand for high-value functional information and improving capacity to deliver.

Proteomic analysis of dental tissues

Teeth are highly refined structures formed by several types of specialised cell. Tooth formation embraces many areas of biomedical interest, including cellular mechanisms for calcium handling, protein secretion and mineralised tissue production. Proteomics offers great potential to elucidate these cellular roles, and to establish their relevance to general cell types. Here we review our proteomic investigations of dental enamel formation, covering both the approaches taken and some findings of general biomedical relevance.

ERp29 is a ubiquitous resident of the endoplasmic reticulum with a distinct role in secretory protein production

ERp29 was recently characterized biochemically as a novel protein that resides in mammalian endoplasmic reticulum (ER). Here we applied immunochemical procedures at the cellular level to investigate the hypothesized role of ERp29 in secretory protein production. ERp29 was localized exclusively to the ER/nuclear envelope of MDCK cells using confocal immunocytochemistry and comparative markers of the ER lumen, ER/Golgi membrane, nuclei, and mitochondria. A predominant association with rough ER was revealed by sucrose-gradient analysis of rat liver microsomes. Immunohistochemistry showed ERp29 expression in 35 functionally distinct cell types of rat, establishing ERp29 as a general ER marker. The ERp29 expression profile largely paralleled that of protein disulfide isomerase (PDI), the closest relative of ERp29, consistent with a role in secretory protein production. However strikingly different ERp29/PDI ratios were observed in various cell types, suggesting independent regulation and functional roles. Together, these findings associate ERp29 primarily with the early stages of secretory protein production and implicate ERp29 in a distinct functional role that is utilized in most cells. Our identification of several ERp29-enriched cell types suggests a potential selectivity of ERp29 for non-collagenous substrates and provides a physiological foundation for future investigations.

Endoplasmic reticulum Ca2+-ATPase pump is up-regulated in calcium-transporting dental enamel cells: a non-housekeeping role for SERCA2b

Dental enamel-forming cells face a major challenge to avoid the cytotoxic effects of excess calcium. We have characterized sarcoplasmic/endoplasmic reticulum calcium-ATPase pumps (SERCA) in rat enamel cells to address the proposal that non-mitochondrial calcium stores play a dominant role in transcellular calcium transport. A single major isoform, SERCA2b, was detected during the protein-secretory and calcium-transport stages of enamel formation using reverse-transcriptase PCR, cDNA cloning, Northern analysis and immunoblotting. Most importantly, SERCA2b exhibited a specific 3-fold up-regulation to high expression levels during calcium transport, as determined by quantitative immunoblotting and ATPase assays. Sensitivity of the calcium-dependent ATPase to thapsigargin and three other SERCA inhibitors was characterized. These findings indicate that enamel cells are well-equipped to sequester calcium in endoplasmic reticulum stores and so protect against calcium toxicity, associate SERCA with transcellular calcium transport for the first time, and establish SERCA2b as a molecular and pharmacological target for future investigations of calcium transcytosis. The observed physiological regulation in enamel cells contradicts the widespread perception that SERCA2b is restricted to general housekeeping duties.

Evaluation and management of the patient co-infected with human immunodeficiency virus and hepatitis C

The emerging presence of hepatitis C viral (HCV) infection in the United States has been the focus of much attention among health care providers and the general population. Among patients infected with human immunodeficiency virus (HIV), there has been a dramatic increase in hepatitis C disease. During the 1980s and early 1990s, hepatitis C was viewed as a disease for which little could be done, both because of ineffective treatment and the severity and lack of adequate treatments for acquired immune deficiency syndrome (AIDS) itself. Treatment with interferon had poor effect on hepatitis C in the co-infected population, especially for those with advanced immunosuppression. The regimen was difficult to tolerate even with dose reductions. With the advent of highly active antiretroviral therapy (HAART) and effective treatment and prophylaxis for opportunistic infections, a substantial portion of HIV-infected patients are living long enough to have their health compromised by hepatic failure or hepatocellular carcinoma owing to hepatitis C, rather than by AIDS-related illness. New treatments are available for hepatitis C, with preliminary research yielding promising results. The role of these medications in managing HIV/HCV co-infection is currently under study, with implications for many. Health care providers are increasingly faced with the challenges of caring for people infected with the hepatitis C virus, and the growing number of individuals co-infected with hepatitis C and HIV. The purpose of this article is to provide an overview of hepatitis C, especially in the presence of HIV infection, and to detail the recognition and management of the care of this emerging population.

Human ERp29: isolation, primary structural characterisation and two-dimensional gel mapping

Recently we characterised a novel 29 kDa endoplasmic reticulum protein that is widely expressed in rat tissues, and named it ERp29. Several ERp29-like gene products have been reported in human tissues but uncertainty surrounds their relationships with each other and rat ERp29. To clarify these issues, ERp29 was isolated from human liver and characterised by primary structural analysis and two-dimensional gel mapping. Comparisons with rat ERp29 revealed striking homologies both in sequence and physical properties. Characterisation of the isoelectric heterogeneity and anomalous mass on two-dimensional gels enabled two reported homologues (UL35 and ERp31) to be identified as ERp29. Resolution of a sequence discrepancy led to unequivocal correlation of human ERp29 with the cognate cDNA previously named ERp31 and ERp28. Consequent links established to human genome and proteome projects showed that ERp29 is encoded by a gene on chromosome 12 that is expressed universally in human tissues. Together, these findings unified various ERp29 homologues as products of a single gene orthologous to rat ERp29 and established ERp29 as the only known member of a new protein class. Investigations of ERp29 function in human health and disease should benefit from the integrated links between genome, proteome and murine model organisms established here.

Calcium transport across the dental enamel epithelium

Dental enamel is the most highly calcified tissue in mammals, and its formation is an issue of fundamental biomedical importance. The enamel-forming cells must somehow supply calcium in bulk yet avoid the cytotoxic effects of excess calcium. Disrupted calcium transport could contribute to a variety of developmental defects in enamel, and the underlying cellular machinery is a potential target for drugs to improve enamel quality. The mechanisms used to transport calcium remain unclear despite much progress in our understanding of enamel formation. Here, current knowledge of how enamel cells handle calcium is reviewed in the context of findings from other epithelial calcium-transport systems. In the past, most attention has focused on approaches to boost the poor diffusion of calcium in cytosol. Recent biochemical findings led to an alternative proposal that calcium is routed through high-capacity stores associated with the endoplasmic reticulum. Research areas needing further attention and a working model are also discussed. Calcium-handling mechanisms in enamel cells are more generally relevant to the understanding of epithelial calcium transport, biomineralization, and calcium toxicity avoidance.

Enamel cell biology. Towards a comprehensive biochemical understanding

Enamel cells ultimately determine the properties of dental enamel. Surprisingly little is known about enamel cell functions at the biochemical and molecular levels. Understanding of both normal and abnormal enamel formation should benefit from elucidation of this area. This paper reviews our recent efforts to establish microscale biochemical analyses of rat enamel cells, and the ensuing initial findings about their protein phenotype (i.e., proteome) and calcium-handling mechanisms. A perspective of the current status of enamel cell research, and where it might head, is also given.

ToothPrint, a proteomic database for dental tissues

Increasing demand exists to disseminate and integrate proteomic data as proteome analysis assumes a commanding role in the postgenome era. Databases on the World Wide Web are an effective means to share information obtained from two-dimensional gels and allied proteomic approaches. Here we report the establishment of ToothPrint, a proteomic database for dental tissues accessed at http://toothprint.otago.ac.nz. Using developing rat enamel as a prototype, ToothPrint provides a variety of functionally relevant data (ligand binding, subcellular localisation, developmental regulation) in addition to protein identification maps. Features designed to enhance usability of the website and simplify its computing requirements are also outlined. Customized for mineralizing tissues, ToothPrint should prove to be an effective bioinformatic resource for investigations of dental biology.

Human ERp29: isolation, primary structural characterisation and two-dimensional gel mapping

Recently we characterised a novel 29 kDa endoplasmic reticulum protein that is widely expressed in rat tissues, and named it ERp29. Several ERp29-like gene products have been reported in human tissues but uncertainty surrounds their relationships with each other and rat ERp29. To clarify these issues, ERp29 was isolated from human liver and characterised by primary structural analysis and two-dimensional gel mapping. Comparisons with rat ERp29 revealed striking homologies both in sequence and physical properties. Characterisation of the isoelectric heterogeneity and anomalous mass on two-dimensional gels enabled two reported homologues (UL35 and ERp31) to be identified as ERp29. Resolution of a sequence discrepancy led to unequivocal correlation of human ERp29 with the cognate cDNA previously named ERp31 and ERp28. Consequent links established to human genome and proteome projects showed that ERp29 is encoded by a gene on chromosome 12 that is expressed universally in human tissues. Together, these findings unified various ERp29 homologues as products of a single gene orthologous to rat ERp29 and established ERp29 as the only known member of a new protein class. Investigations of ERp29 function in human health and disease should benefit from the integrated links between genome, proteome and murine model organisms established here.

Calbindin28kDa is specifically associated with extranuclear constituents of the dense particulate fraction

Recent attempts to understand the function of calbindin28kDa, a widely expressed calcium-binding protein, are confounded by uncertainties over its subcellular location. Using immunoblot analysis of rat brain subregions, we found that the proportion of particulate calbindin28kDa (24-43% of total) was independent of expression level and location. The association of calbindin28kDa with particulate structures appeared to be specific, since it persisted when soluble calbindin28kDa was sequestered by antibodies added before tissue disruption. Moreover, when exogenous calbindin28kDa was added during homogenisation of brain from calbindin28kDa-nullmutant mice, only 10% partitioned to the particulate fraction compared with 33% of endogenous calbindin28kDa in wildtype controls. Confocal microscopy showed that calbindin28kDa was predominantly extranuclear in all tissues analysed (i.e. various brain regions, isolated neurons, and dental enamel epithelium). Dual-label microscopy of neural dense particulate fractions confirmed the extranuclear location of calbindin28kDa and also showed that it partly colocalised with synaptosome and microtubule markers. Using sucrose step gradients, calbindin28kDa was separated from nuclei in parallel with synaptosome and endoplasmic reticulum markers. However, no association with the marker proteins (synaptophysin, ERp29, alpha/beta-tubulin) was detected by calbindin28kDa-immunoprecipitation analysis. Together these findings provide the first consistent picture that calbindin28kDa is located predominantly outside of the nucleus, irrespective of tissue type (neuronal vs. non-neuronal) and experimental approach (biochemical vs morphological). The evidence of a substantial, strong and specific association with insoluble cellular structures challenges the widely held view of calbindin28kDa as a mobile calcium buffer, and supports the existence of important alternative roles that involve target proteins.

Isolation of ERp29, a novel endoplasmic reticulum protein, from rat enamel cells. Evidence for a unique role in secretory-protein synthesis

Recently we cloned and described ERp29, a novel 29-kDa endoplasmic reticulum (ER) protein that is widely expressed in rat tissues. Here we report our original isolation of ERp29 from dental enamel cells, and the comprehensive sequence analysis that correlated ERp29 with its cognate cDNA, both in enamel cells and liver. Fractionation of enamel cells using a new freeze-thaw procedure showed that ERp29 partitioned with known reticuloplasmins, and not with soluble proteins from mitochondria or cytosol. The absence of ERp29 in secreted enamel matrix indicated that the C-terminal tetrapeptide (KEEL motif) confers effective ER-retention in enamel cells. ERp29 behaved as a single species (approximately 40 kDa) during size-exclusion chromatography of liver reticuloplasm, suggesting that most ERp29 is not stably associated with other proteins. Immunoblot analysis showed that ERp29 was up-regulated during enamel secretion and expressed most highly in secretory tissues, indicative of a role in secretory-protein synthesis. Unlike other reticuloplasmins, ERp29 was down-regulated during enamel mineralization and thereby dissociated from a calcium-handling role. Tissue-specific variations in ERp29 molecular abundance were revealed by quantification of reticuloplasmin mole ratios.

IN CONCLUSION

(a) ERp29 is a novel reticuloplasmin of general functional importance; (b) a unique role in protein processing is implicit from the distinctive expression patterns and molecular structure; (c) ERp29 is primarily involved in normal protein secretory events, not the ER stress response; (d) a major role is likely in tissues where ERp29 was equimolar with established molecular chaperones and foldases. This study implicates ERp29 as a new member of the ER protein-processing machinery, and identifies tissues where the physiological role of ERp29 is most likely to be clearly manifested.

Proteomic analysis of enamel cells from developing rat teeth: big returns from a small tissue

Poor understanding of the molecular links between disturbed calcium regulation in cells and disease remains a problem of considerable biomedical importance. Dental enamel cells might provide useful insights to this problem since they handle calcium in bulk without suffering its cytotoxic effects. Two practical challenges hindered investigation of calcium handling mechanisms in enamel cells--paucity of molecular information and limited availability of sample from developing rat teeth, the experimental system of choice. This paper outlines the microscale proteomic approaches we applied to overcome these difficulties and reviews several major findings that ensued from initial characterization of the enamel cell proteome. Enamel cells are now established as a powerful model for fundamental calcium research and have provided outcomes of broad biological relevance, including the discovery of a new endoplasmic reticulum protein. Future proteomic approaches that might benefit understanding of function are discussed.

Distribution of Borrelia burgdorferi s.l. spirochaete DNA in British ticks (Argasidae and Ixodidae) since the 19th century, assessed by PCR

The distribution of Borrelia burgdorferi sensu lato, the Lyme borreliosis agent, was surveyed in British ticks in the collection of the Natural History Museum, London. Alcohol-preserved specimens of eight species of ticks known to attack humans were studied: Ixodes ricinus, I. hexagonus, I. uriae, I. trianguliceps, Dermacentor reticulatus, Haemaphysalis punctata, Rhipicephalus sanguineus and Argas vespertilionis. The sample comprised all life stages and originated from a wide range of host species, collection dates (1896-1994) and geographical localities in England, Scotland and Wales. Borrelia burgdorferi s.l. DNA, detected by a polymerase chain reaction that targeted the outer surface protein A gene, was found in all eight species. The overall proportion of PCR-positive specimens ranged from 7.8% for I. hexagonus (mostly from mustelids and hedgehogs) to 98.3% for I. uriae (mostly from seabirds). Borrelia burgdorferi s.l. DNA was found for the first time in the bat parasite A. vespertilionis (85.3%). The spirochaete is newly recorded in British populations of I. trianguliceps (97.4%, mostly from voles, mice and shrews), D. reticulatus (12.5% from dog and man) and R. sanguineus (30% from dogs and human dwellings). Of the four tick species with larvae available for testing, examples of I. ricinus, I. uriae and A. vespertilionis were PCR positive, as were significantly more nymphs than adults of I. ricinus, I. hexagonus and A. vespertilionis. Analyses showed that B. burgdorferi s.l. has been consistently present in British tick populations since at least 1897. Ticks positive for B. burgdorferi s.l. DNA were collected in all months of the year, throughout Britain, and were found on a wide range of mammal and bird species. PCR positivity does not prove vector or reservoir competence, but the use of archived material has demonstrated an extensive range of host-tick relationships involving B. burgdorferi s.l. in Britain for > 100 years.

Lysozyme and alpha-lactalbumin from the milk of a marsupial, the common brush-tailed possum (Trichosurus vulpecula)

Lysozyme and alpha-lactalbumin have been identified using N-terminal sequence analysis of whey proteins from the common brush-tailed possum, Trichosurus vulpecula after separation by two-dimensional denaturing electrophoresis. Both proteins were purified from pooled possum milk using ion exchange chromatography and gave mass values of 14,896 and 13,985 Da respectively by MALDI-TOF mass spectrometry. Clones containing the full coding sequences of the genes for both proteins were isolated from a possum mammary cDNA library and the DNA sequence of the coding region determined. The inferred protein sequences were used in phylogenetic analysis of both protein classes. These showed that the T. vulpecula alpha-lactalbumin, along with other marsupial alpha-lactalbumins, formed a family distinct from the eutherian alpha-lactalbumins and the alpha-lactalbumin of a monotreme, the platypus, consistent with the separate evolution of the marsupials. By contrast the T. vulpecula lysozyme was shown to be similar to the ruminant stomach lysozymes and primate lysozymes and quite distinct from the Ca2+-binding lysozymes found in the milk of the echidna and horse.

Molecular cloning of ERp29, a novel and widely expressed resident of the endoplasmic reticulum

We have isolated a full-length cDNA clone for a novel 29 kDa protein that is highly expressed in rat enamel cells. The clone encodes a 259-residue protein, here named ERp29, with structural features (signal peptide and a variant endoplasmic reticulum-retention motif, KEEL) that indicate it is a reticuloplasmin. ERp29 has limited homology with protein disulfide isomerase and its cognates, but lacks their characteristic thioredoxin-like catalytic moiety and calcium-binding motifs. ERp29 mRNA was expressed in all rat tissues tested, and a homologous transcript was detected in other animal livers (primate, ruminant, marsupial). In human hepatoma cells, ERp29 mRNA expression was not increased by stresses (tunicamycin, calcium ionophore) that induced other reticuloplasmins. We conclude that ERp29 is a new, highly conserved member of the reticuloplasmin family which is widely expressed. The apparent lack of both calcium binding properties and stress responsiveness distinguish ERp29 from all major reticuloplasmins characterised to dates.

Mitochondrial ATP synthase F1-beta-subunit is a calcium-binding protein

Mitochondrial ATP synthase is responsive to changes in cytosolic calcium concentration, but the regulatory mechanisms are unclear. Here we identified a major 52 kDa calcium-binding protein in rat enamel cells as the mitochondrial ATP synthase F1-beta-subunit. The F1-beta-subunit behaved as a low affinity and moderate capacity calcium-binding protein during comparative 45Ca overlay analyses. Equivalent behavior was shown by the F1-beta-subunit from rat liver mitochondria, but not by the homologous F1-alpha-subunit, supporting the specificity of calcium binding. Evidence that the catalytic F1-beta-subunit binds calcium specifically introduces new mechanistic possibilities for regulating ATP synthase, and thereby coordinating ATP production with demand for ATP-fuelled calcium pump activity.

Abundant calcium homeostasis machinery in rat dental enamel cells. Up-regulation of calcium store proteins during enamel mineralization implicates the endoplasmic reticulum in calcium transcytosis

Enamel cells handle large amounts of calcium, particularly during the developmental phase (termed maturation) when dental enamel is hypermineralized. The extent of intracellular calcium burden, and the nature of calcium homeostasis machinery used to accommodate it, are largely unknown. Here, the calcium-binding capacity of enamel cell cytosol was found to increase during development, in parallel with the putative transcellular flux of calcium. At maturation, the abundance of calcium-binding proteins in enamel cells exceeded that in brain and other established calcium-oriented tissues, which implies a large calcium burden. A search for likely cytosolic calcium transporters revealed only one high-affinity calcium-binding protein (12 kDa, distinguished from alpha-parvalbumin) that was up-regulated during maturation, but its low abundance (0.02% of soluble protein) precluded a major calcium transport or cytoprotective role. Two low-affinity calcium-binding proteins up-regulated during maturation (by 1.8-fold and 2.1-fold respectively) were identified as calreticulin and endoplasmin, both residents of the endoplasmic reticulum. Together, calreticulin and endoplasmin constituted an exceptionally high proportion (5%) of soluble protein during maturation, which gives an inferred calcium capacity 67-fold higher than that of the principal cytosolic calcium-binding protein. 28-kDa calbindin. Evidence that endoplasmin expression varied inversely with serum calcium concentration, and that the inositol trisphosphate receptor also was highly expressed during maturation, supported the novel hypothesis that non-mitochondrial calcium stores play a major role in transcellular calcium transport.

IN CONCLUSION

(a) enamel cells contain a general high abundance of calcium homeostasis proteins, consistent with a heavy intracellular calcium burden; (b) the expression pattern (phenotype) of calcium-binding proteins varies with enamel cell function; (c) enamel cells appear to contain unusually large non-mitochondrial calcium stores; (d) contrary to the prevailing view that calcium passes mainly through the cytosol of calcium-transporting cells, the findings imply a route through the endoplasmic reticulum. This study gives novel information about how a highly calcium-oriented tissue avoids calcium toxicity, and provides a new focus for investigations into the mechanisms of transcellular calcium transport.

Articular debridement versus washout for degeneration of the medial femoral condyle. A five-year study

In a prospective randomised trial 76 knees with isolated degenerative changes in the medial femoral condyle of grades 3 or 4 were treated by either arthroscopic debridement (40) or washout (36). All knees were followed up for at least one year and 58 for five years. The mean follow-up time was 4.5 years in the debridement group and 4.3 years in the washout group. At one year 32 of the debridement group and five of the washout group were painfree and at five years 19 of a total of 32 survivors in the debridement group and three of the 26 in the washout group were also free from pain. The mean improvement in a modified Lysholm score was 28 for the debridement group at one year and 21 at five years. In the washout group it was only 5 at one year and 4 at five years. For knees with lesions of the medial femoral condyle of grades 3 or 4, arthroscopic debridement appears to be the treatment of choice with over half the patients free from pain after five years.

Calbindin28kDa and calbindin30kDa (calretinin) are substantially localised in the particulate fraction of rat brain

Calbindin28kDa is implicated in cytosolic calcium transport and calciprotection functions, principally as a mobile calcium buffer. Using immunoblotting, we have found that 36% of total calbindin28kDa is in the particulate fraction of rat brain. Particulate calbindin28kDa was located both within and outside organelles and required detergent for solubilisation. Equivalent observations were made for calbindin30kDa, 27% of which was insoluble. These findings indicate that a substantial proportion of calbindin does not function as a mobile calcium buffer, and perhaps instead has a calcium signalling role through target ligands in the insoluble cellular fraction.