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In the developing muscle, a pool of myogenic progenitor cells is formed and maintained. These resident progenitors provide a source of cells for muscle growth in development and generate satellite cells in the perinatal period. By the use of conditional mutagenesis in mice, we demonstrate here that the major mediator of Notch signaling, the transcription factor RBP-J, is essential to maintain this pool of progenitor cells in an undifferentiated state. In the absence of RBP-J, these cells undergo uncontrolled myogenic differentiation, leading to a depletion of the progenitor pool. This results in a lack of muscle growth in development and severe muscle hypotrophy. In addition, satellite cells are not formed late in fetal development in conditional RBP-J mutant mice. We conclude that RBP-J is required in the developing muscle to set aside proliferating progenitors and satellite cells.yogenesis is a tightly regulated process that is essential in muscle development and regeneration. During mammalian development, phases of embryonic and fetal myogenic differentiation lead to the formation and growth of skeletal muscles. In the postnatal and adult organism, skeletal muscle grows and regenerates by the myogenic differentiation of stem cells, the satellite cells (1). Muscle progenitor cells during development or satellite cells in the adult initiate myogenic differentiation as a result of the activation of myogenic determination factors like Myf5 and MyoD and form myoblasts (for reviews, see refs. 2-4). Mononucleated myoblasts begin to express muscle-specific proteins and fuse to form multinucleated myotubes, the constituents of mature skeletal muscle.Skeletal muscle and satellite cells of the body and the extremities derive from the somites, segmental derivatives of the paraxial mesoderm (5-10). As the somite matures, myogenic progenitor cells become confined to the dermomyotome that expresses the transcription factor Pax3 (paired box protein 3). After myogenesis is initiated, a resident progenitor population that expresses Pax3 and Pax7 is maintained in the developing muscle (7-9). Late in fetal development, the progenitor population generates satellite cells, which are marked by the expression of Pax7 (7-9). Some, but not all, satellite cells also express Pax3 (11). Thus, in a developing or adult muscle, a pool of undifferentiated cells is preserved that has the potential to undergo myogenic differentiation. The molecular mechanism used to set aside this population of progenitor cells is not understood.The Notch signaling pathway is highly conserved in evolution and plays important roles during development and in the adult. Notch signals regulate diverse processes, including maintenance of progenitors, cell fate decisions, proliferation, and differentiation (for reviews, see refs. 12-14). Notch signaling is initiated by the interaction of the Notch receptor (Notch 1-4 in mammals) with its ligand (Delta-like 1, 3, and 4 and Jagged 1 and 2 in mammals). Ligand binding results in proteolytic cleavage of the receptor and ...

A novel function of NF-B in the development of most ectodermal appendages, including two types of murine pelage hair follicles, was detected in a mouse model with suppressed NF-B activity (c IB␣⌬N ). However, the developmental processes regulated by NF-B in hair follicles has remained unknown. Furthermore, the similarity between the phenotypes of c IBA⌬N mice and mice deficient in Eda A1 (tabby) or its receptor EdaR (downless) raised the issue of whether in vivo NF-B regulates or is regulated by these novel TNF family members. We now demonstrate that epidermal NF-B activity is first observed in placodes of primary guard hair follicles at day E14.5, and that in vivo NF-B signalling is activated downstream of Eda A1 and EdaR. Importantly, ectopic signals which activate NF-B can also stimulate guard hair placode formation, suggesting a crucial role for NF-B in placode development. In downless and c IB␣⌬N mice, placodes start to develop, but rapidly abort in the absence of EdaR/NF-B signalling. We show that NF-B activation is essential for induction of Shh and cyclin D1 expression and subsequent placode down growth. However, cyclin D1 induction appears to be indirectly regulated by NF-B, probably via Shh and Wnt. The strongly decreased number of hair follicles observed in c IB␣⌬N mice compared with tabby mice, indicates that additional signals, such as TROY, must regulate NF-B activity in specific hair follicle subtypes.

ObjectiveSignal hyperintensity on unenhanced MRI in certain brain regions has been reported after multiple administrations of some, but not all, gadolinium-based contrast agents (GBCAs). One potential initial pathway of GBCA entry into the brain, infiltration from blood into the cerebrospinal fluid (CSF), was systematically evaluated in this preclinical study.MethodsGBCA infiltration and distribution in the CSF were investigated in healthy rats using repeated fluid-attenuated MRI up to 4 h after high-dose (1.8 mmol/kg) administration of six marketed and one experimental GBCA. Additionally, gadolinium measurements in CSF, blood and brain tissue samples (after 24 h) were performed using inductively coupled plasma mass spectrometry.ResultsEnhanced MRI signals in the CSF spaces with similar distribution kinetics were observed for all GBCAs. No substantial differences in the gadolinium concentrations among the marketed GBCAs were found in the CSF, blood or brain tissue. After 4.5 h, the concentration in the CSF was clearly higher than in blood but was almost completely cleared and lower than the brain tissue concentration after 24 h.ConclusionsIn contrast to the brain signal hyperintensities, no differences in penetration and distribution into the CSF of healthy rats exist among the marketed GBCAs.Key Points• Gadolinium-based contrast agents can cross the blood-CSF barrier.• Fluid-attenuated MRI shows GBCA distribution with CSF flow.• GBCA structure and physicochemical properties do not impact CSF penetration and distribution.• GBCA clearance from CSF was almost complete within 24 h in rats.• CSF is a potential pathway of GBCA entry into the brain.

ObjectivesIn this prospective preclinical study, we evaluated T1-weighted signal intensity in the deep cerebellar nuclei (CN) and globus pallidus (GP) up to 24 days after repeated administration of linear and macrocyclic gadolinium-based contrast agents (GBCAs) using homologous imaging and evaluation methods as in the recently published retrospective clinical studies. In a second part of the study, cerebrospinal fluid (CSF) spaces were evaluated for contrast enhancement by fluid-attenuated magnetic resonance imaging (MRI).Materials and MethodsSixty adult male Wistar-Han rats were randomly divided into a control and 5 GBCA groups (n = 10 per group). The administered GBCAs were gadodiamide, gadopentetate dimeglumine, and gadobenate dimeglumine (linear GBCAs) as well as gadobutrol and gadoterate meglumine (macrocyclic GBCAs) and saline (control). Over a period of 2 weeks, the animals received 10 intravenous injections at a dose of 2.5 mmol Gd/kg body weight, each on 5 consecutive days per week. Before GBCA administration, as well as 3 and 24 days after the last injection, a whole-brain MRI was performed using a standard T1-weighted 3-dimensional turbo spin echo sequence on a clinical 1.5 T scanner. The ratios of signal intensities in deep CN to pons (CN/Po) and GP to thalamus (GP/Th) were determined. For the evaluation of the CSF spaces, 18 additional rats were randomly divided into 6 groups (n = 3 per group) that received the same GBCAs as in the first part of the study. After MR cisternography for anatomical reference, a fluid-attenuated inversion recovery sequence was performed before and 1 minute after intravenous injection of a dose of 1 mmol Gd/kg body weight GBCA or saline.ResultsA significantly increased signal intensity ratio of CN/Po was observed 3 and 24 days after the last injection of gadodiamide and gadobenate dimeglumine. No significant changes were observed between the 2 time points. Gadopentetate dimeglumine injection led to a moderately elevated but statistically not significant CN/Po signal intensity ratio. No increased CN/Po signal intensity ratios were determined in the MRI scans of rats that received macrocyclic GBCAs gadobutrol and gadoterate meglumine or saline. The ratio of signal intensity in GP/Th was not elevated in any group injected with GBCAs or saline. Enhanced signal intensities of CSF spaces were observed in the postcontrast fluid-attenuated inversion recovery images of all animals receiving GBCAs but not for saline.ConclusionsIn this animal study in rats, increased signal intensity in the CN was found up to 24 days after multiple, extended doses of linear GBCAs. However, in contrast to clinical reports, the signal enhancement in the GP was not reproduced, demonstrating the limitations of this animal experiment. The elevated signal intensities remained persistent over the entire observation period. In contrast, no changes of signal intensities in either the CN or the GP were observed for macrocyclic GBCAs. However, all GBCAs investigated were able to pass the blood-CSF barrier in ra...

Iodinated contrast media (CM) can induce acute kidney injury (AKI). CM share common iodine-related cytotoxic features but differ considerably with regard to osmolality and viscosity. Meta-analyses of clinical trials generally failed to reveal renal safety differences of modern CM with regard to these physicochemical properties. While most trials' reliance on serum creatinine as outcome measure contributes to this lack of clinical evidence, it largely relies on the nature of prospective clinical trials: effective prophylaxis by ample hydration must be employed. In everyday life, patients are often not well hydrated; here we lack clinical data. However, preclinical studies that directly measured glomerular filtration rate, intrarenal perfusion and oxygenation, and various markers of AKI have shown that the viscosity of CM is of vast importance. In the renal tubules, CM become enriched, as water is reabsorbed, but CM are not. In consequence, tubular fluid viscosity increases exponentially. This hinders glomerular filtration and tubular flow and, thereby, prolongs intrarenal retention of cytotoxic CM. Renal cells become injured, which triggers hypoperfusion and hypoxia, finally leading to AKI. Comparisons between modern CM reveal that moderately elevated osmolality has a renoprotective effect, in particular, in the dehydrated state, because it prevents excessive tubular fluid viscosity.

Notch genes encode cell surface proteins, which are evolutionary conserved and found in invertebrates like Drosophila melanogaster as well as in all vertebrate species. The transcription factor RBP-J (Rbpsuh) is a primary nuclear mediator of Notch signals. Signals provided by Notch receptors control cell fate decisions, patterning, and they also affect proliferation or the maintenance of progenitor cells. In these Perspectives we highlight the recent findings on the role of Notch/RBP-J signaling in the maintenance of muscle progenitor cells during embryogenesis and in the generation of satellite cells in fetal development.

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