Microbial interactions with host molecules, and programmed responses to host environmental stimuli, are critical for colonization and initiation of pathogenesis. Bacteria of the genus Streptococcus are primary colonizers of the human mouth. They express multiple cell-surface adhesins that bind salivary components and other oral bacteria and enable the development of polymicrobial biofilms associated with tooth decay and periodontal disease. However, the mechanisms by which streptococci invade dentine to infect the tooth pulp and periapical tissues are poorly understood. Here we show that production of the antigen I/II (AgI/II) family polypeptide adhesin and invasin SspA in Streptococcus gordonii is specifically upregulated in response to a collagen type I signal, minimally the tri-peptide Gly-Pro-Xaa (where Xaa is hydroxyproline or alanine). Increased AgI/II polypeptide expression promotes bacterial adhesion and extended growth of streptococcal cell chains along collagen type I fibrils that are characteristically found within dentinal tubules. These observations define a new model of host matrix signal-induced tissue penetration by bacteria and open the way for novel therapy opportunities for oral invasive diseases.

To the Editor: Johannsson et al1 have recently observed upregulation of the Monocarboxylate transporter 1 (MCT1) in congestive heart failure (CHF). The implied increased uptake of exogenous (systemic) lactate confirms the potential significance of increased carbohydrate metabolism as a compensatory adaptation in CHF and also identifies a potential therapeutic focus. However, 3 points deserve emphasis: (1) Although increased MCT-1 appears to be a feature of at least a subgroup of ventricular dysfunction, certainly indicating an increased reliance on systemic lactate, Johannsson et al interpret a lack of comparable glucose transporter (Glut-1 and -4) upregulation to suggest endogenous glycolytically derived lactate is relatively insignificant. Their assertion derives from the widely held traditional notion that fatty acid metabolism (FAM), coincidentally the major energy source in normal hearts, antagonizes glycolysis and vice versa. Contemporary studies, while accepting a relative fuel polarization with mutual FAM/glycolysis antagonism, also emphasize the benefits of synergy between FAM/glycolysis in muscle.2 An example of this synergism is AMP-dependent protein kinase (AMPK), a fundamental orchestrator of myocyte energy regulation. Activation of AMPK, stimulates …

Background — Cardiac metabolism becomes more dependent on carbohydrates in congestive heart failure (CHF), and lactate may be used as an important respiratory substrate. Monocarboxylate transporter 1 (MCT1) promotes cotransport of lactate and protons into and out of heart cells and conceivably flux of lactate between cells, because it is abundantly present in the intercalated disk. Methods and Results — Six weeks after induction of myocardial infarction (MI) in Wistar rats, left ventricular end-diastolic pressures were >15 mm Hg, signifying CHF. MCT1 and connexin43 protein levels in CHF were 260% and 20%, respectively, of those in sham-operated animals (Sham), and the corresponding mRNA signals were 181% and not significantly changed, respectively. Confocal laserscan immunohistochemistry and quantitative immunogold cytochemistry showed that MCT1 density was much higher in CHF than in Sham both at the surface membrane and in the intercalated disk. In CHF, a novel intracellular pool of MCT1 appeared to be associated with cisternae, some close to the T tubules. In contrast, connexin43 particles, seen exclusively at gap junctions, were substantially fewer. Maximum lactate uptake was 107±15 mmol · L −1 · min −1 in CHF and 42±6 mmol · L −1 · min −1 in Sham cells ( P <0.05). The K m values were between 7 and 9 mmol/L ( P =NS). Conclusions — In cardiomyocytes from CHF rats, (1) the amount of functional MCT1 in the sarcolemma, including in the intercalated disk, is increased several-fold; (2) a new intracellular pool of MCT1 appears; (3) another disk protein, connexin43, is much reduced; and (4) increased reliance on lactate and other monocarboxylates (eg, pyruvate) could provide tight metabolic control of high-energy phosphates.

CD147 is a broadly expressed plasma membrane glycoprotein containing two immunoglobulin‐like domains and a single charge‐containing transmembrane domain. Here we use co‐immunoprecipitation and chemical cross‐linking to demonstrate that CD147 specifically interacts with MCT1 and MCT4, two members of the proton‐linked monocarboxylate (lactate) transporter family that play a fundamental role in metabolism, but not with MCT2. Studies with a CD2–CD147 chimera implicate the transmembrane and cytoplasmic domains of CD147 in this interaction. In heart cells, CD147 and MCT1 co‐localize, concentrating at the t‐tubular and intercalated disk regions. In mammalian cell lines, expression is uniform but cross‐linking with anti‐CD147 antibodies caused MCT1, MCT4 and CD147, but not GLUT1 or MCT2, to redistribute together into ‘caps’. In MCT‐transfected cells, expressed protein accumulated in a perinuclear compartment, whereas co‐transfection with CD147 enabled expression of active MCT1 or MCT4, but not MCT2, in the plasma membrane. We conclude that CD147 facilitates proper expression of MCT1 and MCT4 at the cell surface, where they remain tightly bound to each other. This association may also be important in determining their activity and location.

We examined the isoform-specific regulation of monocarboxylate transporter (MCT)1 and MCT4 expression by contractile activity in red and white tibialis anterior muscles. After 1 and 3 wk of chronic muscle stimulation (24 h/day), MCT1 protein expression was increased in the red muscles (+78%, P < 0.05). In the white muscles, MCT1 was increased after 1 wk (+191%) and then was decreased after 3 wk. In the red muscle, MCT1 mRNA accumulation was increased only after 3 wk (+21%; P < 0.05). In the white muscle, MCT1 mRNA was increased after 1 wk (+30%; P < 0.05) and 3 wk (+15%; P < 0.05). MCT4 protein was not altered in either the red or white muscles after 1 or 3 wk. MCT4 mRNA was transiently lowered (approximately 15%) in both muscles in the 1st wk, but MCT4 mRNA levels were back to control levels after 3 wk. In conclusion, chronic contractile activity induces the expression of MCT1 but not MCT4. This increase in MCT1 alone was sufficient to increase lactate uptake from the circulation.

The newly cloned proton-linked monocarboxylate transporter MCT3 was shown by Western blotting and immunofluorescence confocal microscopy to be expressed in all muscle fibers. In contrast, MCT1 is expressed most abundantly in oxidative fibers but is almost totally absent in fast-twitch glycolytic fibers. Thus MCT3 appears to be the major MCT isoform responsible for efflux of glycolytically derived lactic acid from white skeletal muscle. MCT3 is also expressed in several other tissues requiring rapid lactic acid efflux. The expression of both MCT3 and MCT1 was decreased by 40-60% 3 weeks after denervation of rat hind limb muscles, whereas chronic stimulation of the muscles for 7 days increased expression of MCT1 2-3-fold but had no effect on MCT3 expression. The kinetics and substrate and inhibitor specificities of monocarboxylate transport into cell lines expressing only MCT3 or MCT1 have been determined. Differences in the properties of MCT1 and MCT3 are relatively modest, suggesting that the significance of the two isoforms may be related to their regulation rather than their intrinsic properties.