Knowledge regarding the steps and mechanisms related to the intra- and interchain cross-linking of collagen and elastin has evolved steadily during the past 30 years. Recently, effort has been directed at identifying the location and types of cross-links that are found in collagen and elastin. There are two major groups of cross-links: those initiated by the enzyme lysyl oxidase and those derived from nonenzymatically glycated lysine and hydroxylysine residues. The formation of enzymatic cross-links depends on specific enzymes, amino acid sequences, and quaternary structural arrangements. The cross-links that are derived nonenzymatically occur more adventitiously and are important to pathobiological processes. Considerable progress has been made in elucidating the pathways of synthesis for several of the enzymatically mediated cross-links, as well as possible mechanisms regulating the specificity of cross-linking. Although less is known about the chemistry of cross-links arising from nonenzymatically glycated residues, recent progress has also been made in understanding possible biosynthetic pathways and control mechanisms. This review focuses on such progress and hopes to underscore the biological importance of collagen and elastin cross-linking.

Early gestation is critical for placentomal growth, differentiation, and vascularization, as well as fetal organogenesis. The fetal origins of adult disease hypothesis proposes that alterations in fetal nutrition and endocrine status result in developmental adaptations that permanently change structure, physiology, and metabolism, thereby predisposing individuals to cardiovascular, metabolic, and endocrine disease in adult life. Multiparous ewes were fed to 50% (nutrient restricted) or 100% (control fed) of total digestible nutrients from Days 28 to 78 of gestation. All ewes were weighed weekly and diets adjusted for individual weight loss or gain. Ewes were killed on Day 78 of gestation and gravid uteri recovered. Fetal body and organ weights were determined, and numbers, morphologies, diameters, and weights of all placentomes were obtained. From Day 28 to Day 78, restricted ewes lost 7.4% of body weight, while control ewes gained 7.5%. Maternal and fetal blood glucose concentrations were reduced in restricted versus control pregnancies. Fetuses were markedly smaller in the restricted group than in the control group. Further, restricted fetuses exhibited greater right- and left-ventricular and liver weights per unit fetal weight than control fetuses. No treatment differences were observed in any gross placentomal measurement. However, caruncular vascularity was enhanced in conceptuses from nutrient-restricted ewes but only in twin pregnancies. While these alterations in fetal/placental development may be beneficial to early fetal survival in the face of a nutrient restriction, their effects later in gestation as well as in postnatal life need further investigation.

TATA-binding protein (TBP) binds the minor groove of the TATA element with the DNA bent 80 degrees towards the major groove. A constrained minicircle strategy has been used to test the effect of DNA topology on the affinity of TBP for the TATA element. We report here that TBP bound to DNA which was slightly pre-bent towards the major groove with 100-fold higher affinity than unbent (linear) DNA of identical sequence and 300-fold higher affinity than DNA pre-bent towards the minor groove. Similar discrimination was observed with the holo-TFIID transcription complex. DNA topology, particularly bending, is determined by many factors including chromatin in cells and may, through changes in the affinity of the TATA factor, be important in the control of transcription.

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The purpose of this study was to evaluate the effect of endurance exercise training on both locomotor skeletal muscle collagen characteristics and passive stiffness properties in the young adult and old rat. Young (3-mo-old) and senescent (23-mo-old) male Fischer 344 rats were randomly assigned to either a control or exercise training group [young control (YC), old control (OC), young trained (YT), old trained (OT)]. Exercise training consisted of treadmill running at approximately 70% of maximal oxygen consumption (45 min/day, 5 days/wk, for 10 wk). Passive stiffness (stress/strain) of the soleus (Sol) muscle from all four groups was subsequently measured in vitro at 26 degreesC. Stiffness was significantly greater for Sol muscles in OC rats compared with YC rats, but in OT rats exercise training resulted in muscles with stiffness characteristics not different from those in YC rats. Sol muscle collagen concentration and the level of the nonreducible collagen cross-link hydroxylysylpyridinoline (HP) significantly increased from young adulthood to senescence. Although training had no effect on Sol muscle collagen concentration in either age group, it resulted in a significant reduction in the level of Sol muscle HP in OT rats. In contrast, exercise had no effect on HP in the YT animals. These findings indicate that 10 wk of endurance exercise significantly alter the passive viscoelastic properties of Sol muscle in old but not in young adult rats. The coincidental reduction in the principal collagen cross-link HP also observed in response to training in OT muscle highlights the potential role of collagen in influencing passive muscle viscoelastic properties.

The extracellular matrix (EMC) of muscle is composed mostly of the protein collagen with lesser quantities of other constituents such as proteoglycans also present. The focus of this brief review is the extracellular modification of collagen, critical to forming a stable matrix, called crosslinking. Enzyme-mediated covalent collagen crosslinks are largely lysine-derived. Their formation is absolutely essential for stabilization of the EMC and a functional muscle. In cooked meat, the presence of crosslinks contribute to the shrinkage and tension development of collagen as it denatures with a subsequent increase in the toughness of meat. Both crosslink and collagen concentrations vary with differing muscle type, producing a wide range of textural differences among muscles. Furthermore, within a given muscle type, a wide range of conditions, often dependent on management choices, influence crosslinking patterns. Although information regarding the chemical structure, specific location, and quantity of collagen crosslinks is available, mechanisms that control and regulate their formation remain elusive. Recent studies, however, suggest a potential role for the proteoglycan decorin in regulating collagen fibrillogenesis, ordering the spatial arrangement of collagen molecules and, thus, influencing crosslinking patterns.

This study evaluated the single and interactive effects of age and training status on selected collagen parameters in two rodent locomotor skeletal muscles contrasting in fiber type composition. Gastrocnemius (GAST) and soleus (SOL) muscles from both trained (10 wk of daily treadmill running) and sedentary young adult (5-mo-old), middle-aged (15-mo-old), and old (23-mo-old) female Fischer 344 rats were evaluated for concentrations of collagen (measured by hydroxyproline concentration ([OH-Pro])) and of the predominant nonreducible lysine aldehyde-derived collagen cross-link hydroxylysylpyridinoline ([HP]). Maximal aerobic capacity was significantly elevated in all three trained groups compared with sedentary age-matched control groups. Slow-twitch SOL had a significantly higher [OH-Pro] than fast-twitch GAST (P < 0.05). Although aging had no effect on [OH-Pro] in GAST, in SOL a significant increase with age was seen (P < 0.02). In sedentary rats both GAST and SOL [HP] increased with age, with this increase being more pronounced for SOL. Additionally, although training had no effect on the aging-associated increase in GAST [HP], it prevented the rise seen in SOL. The observed training-induced reduction in SOL [HP] presumably reflects exercise recruitment and subsequent stimulation of collagen synthesis and degradation rates in this muscle. We conclude that both aging and training affect the extracellular matrix in rodent limb skeletal muscle.

Maternal obesity (MO) has harmful effects on both fetal development and subsequent offspring health. The impact of MO on fetal myocardium development has received little attention. Fibrogenesis is regulated by the transforming growth factor-β (TGF-β)/p38 signaling pathway. Using the well-established model of MO in pregnant sheep, we evaluated the effect of MO on TGF-β/p38 and collagen accumulation in fetal myocardium. Nonpregnant ewes were assigned to a control diet [Con, fed 100% of National Research Council (NRC) nutrient recommendations] or obesogenic diet (OB, fed 150% of NRC recommendations) from 60 days before conception. Fetal ventricular muscle was sampled at 75 and 135 days of gestation (dG). At 75 dG, the expression of precursor TGF-β was 39.9 ± 9.9% higher (P < 0.05) in OB than Con fetal myocardium, consistent with the higher content of phosphorylated Smad3 in OB myocardium. The phosphorylation of p38 tended to be higher in OB myocardium (P = 0.08). In addition, enhanced Smad complexes were bound to Smad-binding elements in 75 dG OB fetal myocardium measured by DNA mobility shift assay (130.2 ± 26.0% higher, P < 0.05). Similar elevation of TGF-β signaling was observed in OB fetal myocardium at 135 dG. Total collagen concentration in OB was greater than Con fetal myocardium (2.42 ± 0.16 vs. 1.87 ± 0.04%, P < 0.05). Matrix metalloproteinase-9 and tissue inhibitor of metalloproteinase-3 were higher in the Con group compared with OB sheep (43.86 ± 16.01 and 37.23 ± 7.97% respectively, P < 0.05). In summary, MO results in greater fetal heart connective tissue accumulation associated with an upregulated TGF-β/p38 signaling pathway at late gestation; such changes would be expected to negatively impact offspring heart function.