The transcription factor CCAAT enhancer binding protein ␣ (C͞ EBP␣) is expressed at high levels in liver and adipose tissue. Cell culture studies show that C͞EBP␣ is sufficient to trigger differentiation of preadipocytes into mature adipocytes, suggesting a central role for C͞EBP␣ in the development of adipose tissue. C͞EBP␣ knockout mice die within 7-12 h after birth. Defective gluconeogenesis of the liver and subsequent hypoglycemia contribute to the early death of these animals. This short life span impairs investigation of the development of adipose tissue in these mice. To improve the survival of C͞EBP␣؊͞؊ animals, we generated a transgenic line that expresses C͞EBP␣ under the control of the albumin enhancer͞promoter. This line was bred into the knockout strain to generate animals that express C͞EBP␣ in the liver but in no other tissue. The presence of the transgene improved survival of C͞EBP␣؊͞؊ animals almost 3-fold. Transgenic C͞EBP␣؊͞؊ animals at 7 days of age show an absence of s.c., perirenal, and epididymal white fat despite excess lipid substrate in the serum, whereas brown adipose tissue is somewhat hypertrophied and shows minimal biochemical alterations. Interestingly, mammary gland fat tissue is present and exhibits normal morphology. The absence of white adipose tissue in many depots in the presence of high serum lipid levels shows that C͞EBP␣ is required for the in vivo development of this tissue. In contrast, brown adipose tissue differentiation is independent of C͞EBP␣ expression. The presence of lipid in brown adipose tissue serves as an internal nutritional control, indicating that neither nutritional intake nor lipoprotein composition is likely responsible for the absence of white fat.
Cartilage oligomeric matrix protein (COMP) is a pentameric extracellular protein expressed in cartilage and other musculoskeletal tissues. Mutations in the COMP gene cause pseudoachondroplasia (PSACH), a severe dwarfing condition that has a growth plate chondrocyte pathology. PSACH is characterized by intracellular retention of COMP and other extracellular matrix (ECM) proteins, which form an ordered matrix within large rough endoplasmic reticulum cisternae. This accumulation is cytotoxic and causes premature chondrocyte cell death, thereby depleting chondrocytes needed for normal long bone growth. Research to define the underlying molecular mechanisms of PSACH has been hampered by the lack of a suitable model system. In this study, we achieved robust expression of human mutant (MT) or wild-type (WT) COMP in mice by using a tetracycline-inducible promoter. Normal growth plate distribution of ECM proteins was observed in 1-month-old WT-COMP and C57BL\6 control mice. In contrast, the structure of the MT-COMP growth plate recapitulated the findings of human PSACH growth plate morphology, including (1) retention of ECM proteins, (2) intracellular matrix formation in the rER cisternae, and (3) increased chondrocyte apoptosis. Therefore, we have generated the first mouse model to show extensive intracellular retention of ECM proteins recapitulating the human PSACH disease process at the cellular level.
Systolic myocardial velocity and Em are strongly dependent on both the number of myocytes and the myocardial beta-adrenergic receptor density.
Antigen-specific IgG Abs 1 in autoimmune and alloimmune disease are described to catalyze chemical reactions (1-3). Examples of catalytic Abs raised by routine experimental immunization with ordinary antigens have also been published (4 -7). However, no consensus has developed whether naturally occurring catalytic Abs represent rare accidents arising from adaptive sequence diversification processes or genuine enzymes with important functional roles. The major reason is that the turnover (k cat ) of antigen-specific IgG Abs is low. Some catalytic Abs express catalytic efficiencies (k cat /K m ) comparable to conventional enzymes, but this is due to high affinity recognition of the antigen ground state (reviewed in Ref. 8).Certain enzymes cleave peptide bonds by a mechanism involving the formation of a transient covalent intermediate of the substrate and a nucleophilic residue present in the active site. The nucleophiles are generated by intramolecular activation mechanisms, e.g. the activation of Ser/Thr side chain hydroxyl groups by hydrogen bonding to His residues, and can be detected by covalent binding to electrophilic phosphonate diesters (9, 10). Using these compounds as covalently reactive analogs of antigens (CRAs), we observed that IgG Abs express nucleophilic reactivities comparable to trypsin (11). Despite their nucleophilic competence, IgG Abs display low efficiency proteolysis, presumably due to deficiencies in steps occurring after formation of the acyl-Ab intermediate, viz., water attack on the intermediate and product release. Occupancy of the B cell receptor (BCR, surface Ig complexed to ␣ and  subunits along with other signal transducing proteins) by the antigen drives B cell clonal selection. Proteolysis by the BCR is compatible with clonal selection, therefore, only to the extent that the release of antigen fragments is slower than the rate of antigeninduced transmembrane signaling necessary for induction of cell division. Immunization with haptens mimicking the charge characteristics of the transition state (12) has been suggested as a way to surmount the barrier to adaptive improvement of catalytic rate constants. Catalysis by designer IgG Abs obtained by these means, however, also proceeds only slowly.In mice and humans, the initial Ab repertoire consists of ϳ100 heritable VL and VH genes. Adaptive maturational processes expand the repertoire by several orders of magnitude. The initial BCR complex on the pre-B cell surface contains V-(D)-J rearranged Ig chains as a complex with surrogate L chains (reviewed in Ref. 13). Precise assignment of the B cell differentiation stage at which cell division becomes antigen-dependent is somewhat ambiguous, but it is generally believed that non-covalent antigen binding to the pre-BCR is not required for initial cell growth. / chains replace the surrogate L chain at the later stages of antigen-driven B cell differentiation, which is accompanied by diversification via somatic hypermutation processes and continued gene rearrangements (14,15). V-(D)-J gene ...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.