What Do We Know about Biomineralization/Calcification?
Including humans, many multicellular organisms produce similar hard tissues, such as bones, teeth, shells, skeletal units, and spicules. These hard tissues are biocomposites and incorporate both structural macromolecules (lipids, proteins, and polysaccharides) and inorganic minerals.1We do not fully understand the control mechanism of biomineralization in primitive or in developed organisms. The mineral phase of hard tissue is sometimes called biologic apatite, that is, a nonstoichiometric hydroxylapatite. Pure hydroxyapatite has the formula Ca10(PO4)6(OH)2. In contrast, a biologic apatite (like in bone) is nonstoichiometric and contains several other ions, mainly carbonate and other elements in traces such as Mg2+, Na+, Fe2+, HPO4 2−, F−, and Cl−. Consequently, a more appropriate structural formula for the composition of bone is (Ca,X)10(PO4,CO3,Y)6(OH,Z)2, with X substituting cations and Y and Z substituting anions (with the indices 10, 6, and 2 changing according to stoichiometry).2
There is a paradox in medicine. Whereas some researchers have been discussing the cytotoxic effect of apatite in vitro,3,4others have been announcing the safety of in vivo apatite applications.5-8Although these disagreements have not been completely resolved, both biogenic and nonbiogenic apatite materials have been continuously used in drug delivery and transplantation.6,9We know that when apatite is found in soft tissue, it is considered to be pathologic calcification.10The causes of apatite-deposit formations in soft tissue have been discussed for decades but still remain speculative. For example, calcification in the coronary arteries has been widely regarded as an uncommon, end-stage, insignificant, passive, degenerative process of aging-a notion that has paralyzed research in this area for decades.11Interestingly, these same terms were once used to describe atherosclerosis.11Today, we know that coronary artery calcification occurs, almost exclusively, at sites of atherosclerotic lesions.12Calcification in the development of these plaques is a complicated, actively regulated process of mineralization that is similar to bone formation and remodeling.13,14Mineralogists explain that all that is needed for crystal formation/biomineralization to start is nidi (nucleus) and an environment of available dissolved components at or near saturation concentrations, along with the absence of inhibitors for crystal formation.15Bacteria or other agents producing such nidi, if present in blood and in urine, are very likely candidates to launch and accelerate pathologic calcification in vivo.16,17This is clinically important because blood contains phosphate near its saturation level.18
“Nanobacteria”: Potential Nidi for Calcification, a Good Model for Studying Calcification Mechanism
What is known about microbial infections is based on the study of well-known microbes. A poorly known bloodborne agent (discovered and tentatively termed “nanobacteria” [NB] by our team)19exists that behaves as a microbe and appears to show a correlation with such diverse calcification-related health problems as arterial heart disease,20-22Alzheimer's disease,23kidney stone formation,24-28polycystic kidney disease (PKD),29,30gallstones and gallbladder inflammation,31prostatitis,32,33calciphylaxis,34,35and cancer.36,37NB are also called Calcifying Nano Particles or CNP. This agent has unique properties, including an extremely small size (0.1-0.5 μm), as seen in Figure 1. Although the biologic characterization of NB is yet to be fully understood, the precipitation and growth of calcium phosphate readily occur in systems containing trace amounts of NB but not in identical control systems lacking NB.19The exact mechanism(s) by which apatite is nucleated and formed around NB is unknown. When the serum concentration in the medium is reduced (≤ 5%) in the NB culture conditions, NB start to mineralize and grow larger in size owing to calcium and phosphate deposition on their surface (Figure 2). Differential interference contrast microscopy revealed several micrometer-thick age ring-like mineral layers forming around NB in these cultures (see Figure 2B). Chemical analysis using energy-dispersive x-ray (EDX) microanalysis of these mineral layers shows Ca and P peaks.19The effectiveness of NB biomineralization is remarkable: apatite formation in vitro stopped only when the calcium level decreased by 50% from 1.8 to 0.9 mM and the phosphate levels fell to near zero.24Our results indicate that the NB calcium phosphate phase can be formed at pH 7.4, consistent with human physiologic phosphate and calcium concentrations.19,24,38,39This can also be monitored by 85Sr incorporation and provides a unique model for in vitro studies on calcification.24NB-induced apatitic biofilm formation is dependent on the presence of oxygen19,40and can be prevented with several antibiotics and antimetabolites and by high gamma irradiation at sterilizing doses.39,41The apatite produced by NB is biogenic because it is formed in a carbon-containing biomatrix, forms small spherical units of apatite in nanoscale crystal size (that are very resistant to acid hydrolysis), and can be formed at nonsaturating concentrations of calcium and phosphate.19Such spherical units were identified in most of the human kidney stones examined (Figure 3).24,42,43Nonbiogenic apatite has larger crystals that are easily dissolved in acidic solution. Fourier transform infrared spectroscopy (FTIR) of NB revealed the mineral as almost identical to bone mineral (Figure 4). Models for bone formation, which use metastable concentrations of calcium and phosphate, involve gels that include nidi, such as matrix vesicles, apoptotic vesicles, or collagen, but exclude the known proteinaceous inhibitors for crystal formation.44Such systems have not been tested with NB. Vali and colleagues showed that nanoforms contain apatite-protein complexes and immunoelectron microscopy reveals protein antigens in proximity to apatite, suggesting a novel form of protein-associated mineralization.45
In our earlier studies, we examined NB cultures in high-aspect rotating vessels (HARVs) designed at NASA's Johnson Space Center, which are designed to simulate some aspects of microgravity.25NB cultured in HARVs multiplied 4.6 times faster than under stationary conditions and 3.2 times faster than in shaker flask incubation. Interestingly, the results demonstrated that the degree of apatite crystal formation on NB (biomineralization) and the properties of the apatite are strongly affected by the gravity and other specific culture conditions used.25Although some researchers believe that microgravity does not affect crystal formation and biomineralization,46it has been shown that long periods in a microgravity environment do cause loss of bone and enhance kidney stone formation-like biomineralization disorders in astronauts.47-49
In summary, NB is a perfect model for studying biogenic mineralization/calcification because NB (1) are self-replicating particles and have less complicated metabolic pathways, (2) accumulate calcium and phosphate under physiologic conditions, (3) produce a calcium phosphate mineral similar to bone, and (4) exist in physical conditions (eg, pH, gravity, temperature) that are easy to manipulate and that can be replicated for the physiologic model.
Controversy
The first debate about NB revolved around whether these minute particles are alive. To date, critics argue that a particle just 50 to 200 nm in diameter cannot possibly harbor the components necessary to sustain life. Maniloff's work suggests that to contain the deoxyribonucleic acid (DNA) and proteins needed to function, a cell must be at least 140 nm across.50However, recently, it has been shown that a genome constructed to encode 387 protein-coding and 43 structural ribonucleic acid (RNA) genes could sustain a viable synthetic cell, a Mycoplasma laboratorium that can shrink its size below that limitation.51NB are also incredibly resistant to heat and other methods that would normally kill bacteria, which makes some scientists wonder if they might be an unusual form of crystal rather than organisms. Cisar and colleagues presented an alternative theory for the experimental findings of NB scientists. They stated that biomineralization previously attributed to NB may be initiated by nonliving macromolecules and transferred on “subculture” by self-propagating microcrystalline apatite.52
Detection Methods for NB
Methods to diagnose NB in biologics, cells, tissues, blood, and urine include immunodetection with NB-specific monoclonal antibodies, electron microscopy, and culture techniques.39,53Because NB pass through 0.22 μm-pore size filters, which exclude most common microbes, filtration is often used to clean up fluid specimens before culture for NB.53Replication can be measured by particle counting and optical density at 650 nm.41It has also been shown that the growth of the NB could be detected by specific methods, such as enzyme-linked immunosorbent assay, turbidity, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), or methionine and uridine incorporation.19,20Susceptibility tests can be used to test the effects of antibiotics and other chemotherapeutics.19Growth can be prevented with tetracycline, high doses of aminoglycoside antibiotics, ethylenediaminetetraacetic acid (EDTA), cytosine arabinoside, 5-fluorouracil, and gamma irradiation.19
Are NB a Living Entity?
Isolation of any kind of nucleic acid from NB has been difficult, in part because of the mineral surface that they produced during their culture period. They could not be lysed with lysozyme, proteinase K, several other proteinases, lipases, amylases, alkali, ultrasonography, X-press, detergents, or solvents.40We needed to use acid or EDTA-like chelators before the analysis, which were the factors for structural change in nucleic acid. It has been our experience that NB actually inhibit the amplification of added exogenous classic bacterial DNA by polymerase chain reaction (PCR) methods.
In preliminary research, NB cultures incorporated increasing amounts of radiolabeled 35S into macromolecules separable by gel electrophoresis during the 18-day incubation period. Protein pattern detected with Coomassie's stain of SDS-PAGE transferred to an Immobilon membrane (Millipore, Billerica, MA) and autoradiography showed methionine incorporation into several proteins (Figure 5). Miller and colleagues showed that autoradiographs of isolated NB nucleic acid indicated incorporation of uridine into several “nucleic acid” bands.20The negative control, media containing gamma-irradiated serum (ie, inactivated NB), did not yield radioactive bands after exposure to these radiolabeled DNA/RNA and protein precursors. To define the mechanism of incorporation and systematic location of NB, further molecular research is needed.
Are NB Cytotoxic?
Calcified and noncalcified forms of NB have been observed as free, cell-attached, and internalized particles in mammalian cell cultures in vitro.19,53In our experiments, we chose six different fibroblast lines as experimental models because they are the most ubiquitous cells in the animal body and might be most accessible in wound tissue for invading pathogens, with the exception of professional phagocytic leukocytes.53We showed that NB were bound as clusters on the cell surfaces within 15 minutes. It was concluded that NB are internalized either by receptor-mediated endocytosis or by a closely related pathway within 12 hours.53Internalization seems to be necessary for cytotoxicity. We showed that cytotoxicity was dependent on NB concentration and exposure time. Dying cells always contained numerous ingested NB.53Hybridomas and many lymphocytes were found to be affected by NB, but considerably higher doses were needed.40,53The findings that NB are cytotoxic are of interest to nanoparticle toxicity mechanisms in general; similar mechanisms could be used by other nanoparticles for entering cells and causing cytotoxicity.54
In Vivo Effects of NB
Åkerman and colleagues reported that radiolabeled (99mTc) viable NB accumulated in the kidney and appeared in urine after 15 minutes of their intravenous injection into rabbits.55This could be due to the fact that kidneys are the preferred sides for this agent, unlike other known nanoparticles, and the presence of injured epithelium or a nucleus in the kidney or urinary tract provides a preferable niche for NB to adhere and grow, resulting in biocrystallization. A control study in rabbits was performed in which a similar dose of 99mTc-labeled albumin cross-linked particles or tin-technetium nanoparticles as nanocolloids was administered. Nanocolloids were not targeted to kidneys as NB were.55Shiekh and colleagues also observed that NB, when injected intravenously into rats, were localized in kidneys.28In their research, they observed regions of chronic inflammation infiltrated in the cortex and medulla, which could be due to damage induced by NB. The research team has also shown NB adhering to the surface of the epithelium and their penetration into epithelial cells.
NB and Connections to Calcification-Related Diseases
Calcium is the most important intracellular regulator of physiologic responses, is a messenger for hormonal actions, and regulates cell death and multiplication. Additionally, calcium regulates inflammation, blood clotting, immunologic responses, and neural transmission and muscle contraction. In these actions, calcium-binding proteins are the main players. Recently, a large number of calcium-binding proteins have been mapped to be bound on NB.56NB may thus participate in activation-inhibition processes regulating a large number of responses inside and outside cells. Thus, NB could have multiple pathologic actions in the body.
The lifelong prevalence of kidney stones appears to have increased throughout the whole twentieth century and occurs in up to 15% of the population.57Treatment of kidney stones is now estimated to cost over $3,000 per patient per year. The incidence of new cases and recurrences may continue to rise. Thus, new approaches in treatment and prevention could have a huge economic effect apart from benefits in terms of reduced morbidity. Electron microscopic observations show that apatite units produced in serum-free NB cultures are very similar to human apatitic kidney stones (see Figures 2 and 3). Both grow as layers of mineral (see Figure 2) and matrix. Chemical analysis and EDX microanalysis revealed that the composition of this solid mineral formation was similar to that of most extraskeletal tissue calcification and stones.19The crystalline components of kidney stones are calcium oxalate, calcium phosphate, struvite, purines, or cystine.58Fermentor model studies have shown that in kidney stones, calcium phosphate nidi are always formed initially and may subsequently become coated with oxalate or other components.59,60Apparently, apatite may play a key role in the formation of all kidney stones. Our hypothesis underlines the role of active nidi: even supersaturated urine needs nidi for crystallization to appear, and active nidi can make the process more thermodynamically favored so that it could happen apparently outside equilibrium. We have proposed that NB may be active nidi that attach to, invade, and damage the urinary epithelium of collecting ducts and papilla, forming a calcium phosphate center(s).19,24-27,30We found NB in 70 of 72 human kidney stones studied in Finland.24Khullar and colleagues demonstrated 62% culture positivity of NB in stones collected from an Indian population.61Additionally, they showed DNA and distinct protein bands of NB. However, one group has been unsuccessful at replicating NB culture from stones, although they have observed nanoparticles that are morphologically similar to NB in scanning electron microscopy.62
Another urinary disease, PKD, is the most common autosomal dominant lethal disease in humans. There are reports of endotoxin, NB, and fungal antigens and antibodies in human kidney cyst fluids. Interestingly, a higher prevalence of kidney calcifications is observed in PKD than in the normal population.63It has been proposed that the currently known cellular toxicities, tissue distribution, and pharmacology of NB are plausibly related to the known pathology and pharmacology of PKD.29Hjelle and colleagues evaluated 13 PKD cyst fluids and detected NB antigen positivity in each sample as well as in liver cystic fluid from affected individuals.29
Biopsies, urine, and prostatic secretion cultures fail to demonstrate bacterial pathogens more frequently than in asymptomatic controls, yet inflammation or at least inflammatory markers are often detected in chronic prostatitis.64Likewise, the presence of prostatic calculi in younger men is associated with both inflammation and symptoms of chronic pelvic pain syndrome.65,66The core of prostatic calculi is typically calcium apatite,67which is the hallmark of NB action. Wood and Shoskes proposed that there is a potential role of NB in chronic prostatitis.32Recent clinical research targeting these agents has proven effective in treating some patients with refractory category III prostatitis (chronic pelvic pain syndrome).33In that research, NB antigen or antibody was found in 60% of serum and 40% of urine samples. In 10 patients who underwent transrectal ultrasonography after therapy, prostatic stones were decreased in size or resolved in 50%.33
Mechanisms mediating vascular calcification remain incompletely understood. Some have hypothesized the potential role of NB in arterial calcification.20,21,35Miller and colleagues cultured nanosized particles from calcified but not from noncalcified aneurysms.20These particles were stained with NB-specific monoclonal antibodies, recognized by a DNA-specific dye and incorporated radiolabeled uridine, and, after decalcification, they appeared via electron microscopy to contain cell walls.20Therefore, nanometer-scale particles similar to those described as NB isolated from geologic specimens and human kidney stones can be visualized in and cultured from calcified human cardiovascular tissue.20Puskas and colleagues propagated NB-like spherical particles from 26 of 42 sclerotic aorta and carotid samples and confirmed their nature by dot immunoblot by using NB-specific monoclonal antibodies, light microscopy, and transmission electron microscopy.21[3H]L-Aspartic acid was incorporated into high-molecular-weight compounds of demineralized particles. PCR amplification of 16S ribosomal DNA sequences from the particles was unsuccessful using traditional protocols.21Identification of NB-like particles at the lesion supports but does not by itself prove the hypothesis that these agents contribute to the pathogenesis of atherosclerosis, especially vascular calcifications. Specific therapies targeting these particles have demonstrated reduced plaque formation, regression of plaques, and improved lipid profiles.68The potential of anti-NB treatments is controversial and awaits larger clinical trials. Epidemiologic studies have implicated antibodies made by the body against NB to be a strong independent risk factor for coronary artery calcification. The risk appears to be comparable to that of diabetes in the two preliminary studies.69,70The importance of this is that coronary artery calcification is an excellent predictor of future coronary events and death.
Originally, Sedivy and Battistutti reported that NB promoted crystallization of psammoma bodies in ovarian cancer.71Hudelist and colleagues soon verified the 100% concordance between the expression of NB and the presence of psammoma bodies in malignant ovarian tumors. In their research, several lines of evidence suggest the involvement of these organisms in the process of biomineralization. Therefore, they have concluded that NB infection of malignant ovarian tissue contributes to mechanisms leading to the formation of calcified deposits known as psammoma bodies.36
Additionally, NB have been shown to be detected at higher rates in the serum of patients with gallstone disease31and mitral valve calcification.34Others have suggested that NB may contribute to the development of peripheral neuropathy in human immunodeficiency virus (HIV)-positive patients,72,73periodontal problems,22,74and even osteoporosis.75All of these hypothetical approaches require further investigation.
Koch's Postulates
In a small study, García-Cuerpo and colleagues found that translumbar, percutaneous, intrarenal injection of NB (isolated from kidney stones) into rats resulted in kidney stone formation.76Additionally, Shiekh and colleagues examined NB's role in biocrystallization and the in vivo effects on kidney pathology. A calcium oxalate monohydrate assay (COM) was carried out in the presence of NB to study biocrystallization. Wistar rats were given an intravenous injection of NB, and the kidneys were examined for pathologic changes. The COM assay showed accelerated biocrystallization of 14C-oxalate in the presence of NB, indicating them to be efficient candidates for biomineralization. Histopathologic studies revealed bacteria-induced renal tubular calcifications and various manifestations of infection.28Their studies confirm that NB may be involved in the pathogenesis of renal tubular calcification. Such findings are required to prove Koch's postulates linking NB to other pathologic calcification-related diseases.
Conclusion
Whether NB themselves serve as the nucleus for crystal formation or whether the NB are simply able to lower the activation energy barrier and thus allow precipitation and growth of crystals under much lower supersaturation conditions is yet to be determined. However, it is immaterial whether they are bacteria, viruses, or other living or nonliving forms; their properties of promoting ready crystallization and growth of Ca minerals are well established. These self-replicating particles may induce calcification and stone formation in vivo because NB (1) have been detected in human blood, (2) are transported from blood into urine and bile as living organisms, (3) are renotropic, (4) cause apoptotic cell death, (5) are present in human stone isolates and tissues with calcification, and (6) cause kidney stone formation in rats within 1 month when injected in an intrarenal route.
Since NB have been detected in blood and blood products, they should be of interest to the biopharmaceutical industry. For example, recently, NB have been isolated and cultured from the cultured nasopharyngeal carcinoma epithelia HNE1 cell supernatant.77The safety of vaccines produced in cell culture by using (bovine) serum or serum-derived materials in the culture of the cells, sterilized by filtration, is an issue needing thorough risk analysis and method validation.78,79
Only continued research will reveal the nature of NB and their impact on health and disease. NB are a good model system to use in developing drugs to alter the likely diverse pathways involved in tissue calcification. Although the controversy of whether they are living or nonliving entities will continue until new definitive data are collected, this controversy should not overshadow the critical medical importance of understanding the already demonstrated effects of NB on pathologic calcification in the human body and on research into countermeasures to reverse or eliminate these effects.