The melanocortin-4 receptor (MC4-R) is a G protein-coupled, seven-transmembrane receptor expressed in the brain. Inactivation of this receptor by gene targeting results in mice that develop a maturity onset obesity syndrome associated with hyperphagia, hyperinsulinemia, and hyperglycemia. This syndrome recapitulates several of the characteristic features of the agouti obesity syndrome, which results from ectopic expression of agouti protein, a pigmentation factor normally expressed in the skin. Our data identify a novel signaling pathway in the mouse for body weight regulation and support a model in which the primary mechanism by which agouti induces obesity is chronic antagonism of the MC4-R.

Smad proteins are intracellular mediators of signalling initiated by Tgf-betasuperfamily ligands (Tgf-betas, activins and bone morphogenetic proteins (Bmps)). Smads 1, 2, 3, 5 and 8 are activated upon phosphorylation by specific type I receptors, and associate with the common partner Smad4 to trigger transcriptional responses. The inhibitory Smads (6 and 7) are transcriptionally induced in cultured cells treated with Tgf-beta superfamily ligands, and downregulate signalling in in vitro assays. Gene disruption in mice has begun to reveal specific developmental and physiological functions of the signal-transducing Smads. Here we explore the role of an inhibitory Smad in vivo by targeted mutation of Madh6 (which encodes the Smad6 protein). Targeted insertion of a LacZ reporter demonstrated that Smad6 expression is largely restricted to the heart and blood vessels, and that Madh6 mutants have multiple cardiovascular abnormalities. Hyperplasia of the cardiac valves and outflow tract septation defects indicate a function for Smad6 in the regulation of endocardial cushion transformation. The role of Smad6 in the homeostasis of the adult cardiovascular system is indicated by the development of aortic ossification and elevated blood pressure in viable mutants. These defects highlight the importance of Smad6 in the tissue-specific modulation of Tgf-beta superfamily signalling pathways in vivo.

The accumulation of hyperphosphorylated tau in neurofibrillary tangles (NFTs) is a neuropathological hallmark of tauopathies, including Alzheimer's disease (AD) and chronic traumatic encephalopathy, but effective therapies directly targeting the tau protein are currently lacking. Herein, we describe a novel mechanism in which the acetylation of tau on KXGS motifs inhibits phosphorylation on this same motif, and also prevents tau aggregation. Using a site-specific antibody to detect acetylation of KXGS motifs, we demonstrate that these sites are hypoacetylated in patients with AD, as well as a mouse model of tauopathy, suggesting that loss of acetylation on KXGS motifs renders tau vulnerable to pathogenic insults. Furthermore, we identify histone deacetylase 6 (HDAC6) as the enzyme responsible for the deacetylation of these residues, and provide proof of concept that acute treatment with a selective and blood–brain barrier-permeable HDAC6 inhibitor enhances acetylation and decreases phosphorylation on tau's KXGS motifs in vivo. As such, we have uncovered a novel therapeutic pathway that can be manipulated to block the formation of pathogenic tau species in disease.

Scavenger receptor BI (SR-BI) is a cell surface receptor that binds high density lipoproteins (HDL) and mediates selective uptake of HDL cholesteryl esters (CE) in transfected cells. To address the physiological role of SR-BI in HDL cholesterol homeostasis, mice were generated bearing an SR-BI promoter mutation that resulted in decreased expression of the receptor in homozygous mutant (designated SR-BI att) mice. Hepatic expression of the receptor was reduced by 53% with a corresponding increase in total plasma cholesterol levels of 50-70% in SR-BI att mice, attributable almost exclusively to elevated plasma HDL. In addition to increased HDL-CE, HDL phospholipids and apo A-1 levels were elevated, and there was an increase in HDL particle size in mutant mice. Metabolic studies using HDL bearing nondegradable radiolabels in both the protein and lipid components demonstrated that reducing hepatic SR-BI expression by half was associated with a decrease of 47% in selective uptake of CE by the liver, and a corresponding reduction of 53% in selective removal of HDL-CE from plasma. Taken together, these findings strongly support a pivotal role for hepatic SR-BI expression in regulating plasma HDL levels and indicate that SR-BI is the major molecule mediating selective CE uptake by the liver. The inverse correlation between plasma HDL levels and atherosclerosis further suggests that SR-BI may inf luence the development of coronary artery disease.It is well established that plasma concentrations of high density lipoprotein (HDL) cholesterol are inversely proportional to the risk of developing atherosclerosis and coronary artery disease (1). Although the protective mechanism is not known, HDL is thought to reduce plaque formation by removing cholesterol from arterial cells and delivering it to the liver as cholesteryl ester (CE) for bile acid synthesis and secretion, a process referred to as reverse cholesterol transport (2). HDL also delivers CE to steroidogenic tissues (adrenal gland, ovary, and testis), where it serves as substrate for steroid hormone synthesis (reviewed in ref.3). The uptake of HDL cholesterol by cells involves selective transfer of CE to the cell without uptake and degradation of HDL proteins, a process known as selective lipid uptake (4, 5). This is markedly different from the mechanism of clearance of low density lipoproteins (LDL), which involves receptor-mediated endocytosis and intracellular degradation of the entire lipoprotein particle (6).Whereas the receptor that mediates LDL clearance was identified well over a decade ago (6), a functional HDL receptor has only recently been identified. Acton et al. (7) demonstrated that scavenger receptor BI (SR-BI), a multiligand cell surface receptor isolated from Chinese hamster ovary cells by expression cloning (8), binds HDL with high affinity and mediates selective cholesterol uptake in transfected cells. Furthermore, the receptor is primarily expressed in those tissues exhibiting selective lipid uptake in vivo: liver, adrenal gland, ovary, ...

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The aberrant accumulation of tau protein is a pathological hallmark of a class of neurodegenerative diseases known as tauopathies, including Alzheimer's disease and related dementias. On the basis of previous observations that tau is a direct substrate of histone deacetylase 6 (HDAC6), we sought to map all HDAC6-responsive sites in tau and determine how acetylation in a site-specific manner affects tau's biophysical properties in vitro. Our findings indicate that several acetylation sites in tau are responsive to HDAC6 and that acetylation on Lys-321 (within a KCGS motif) is both essential for acetylation-mediated inhibition of tau aggregation in vitro and a molecular tactic for preventing phosphorylation on the downstream Ser-324 residue. To determine the functional consequence of this HDAC6-regulated phosphorylation event, we examined tau's ability to promote microtubule assembly and found that phosphorylation of Ser-324 interferes with the normal microtubule-stabilizing function of tau. Tau phosphorylation of Ser-324 (pSer-324) has not previously been evaluated in the context of tauopathy, and here we observed increased deposition of pSer-324–positive tau both in mouse models of tauopathy and in patients with Alzheimer's disease. These findings uncover a novel acetylation–phosphorylation switch at Lys-321/Ser-324 that coordinately regulates tau polymerization and function. Because the disease relevance of this finding is evident, additional studies are needed to examine the role of pSer-324 in tau pathobiology and to determine whether therapeutically modulating this acetylation–phosphorylation switch affects disease progression in vivo.

The abnormal accumulation of the microtubule-binding protein tau is associated with a number of neurodegenerative conditions, and correlates with cognitive decline in Alzheimer's disease. The ubiquitin ligase carboxy terminus of Hsp70-interacting protein (CHIP) and the molecular chaperone Hsp90 are implicated in protein triage decisions involving tau, and have consequently been targeted for therapeutic approaches aimed at decreasing tau burden. Here, we present evidence that CHIP binds, ubiquitinates and regulates expression of histone deacetylase 6 (HDAC6). As the deacetylase for Hsp90, HDAC6 modulates Hsp90 function and determines the favorability of refolding versus degradation of Hsp90 client proteins. Moreover, we demonstrate that HDAC6 levels positively correlate with tau burden, while a decrease in HDAC6 activity or expression promotes tau clearance. Consistent with previous research on Hsp90 clients in cancer, we provide evidence that a loss of HDAC6 activity augments the efficacy of an Hsp90 inhibitor and drives client degradation, in this case tau. Therefore, our current findings not only identify HDAC6 as a critical factor for the regulation of tau levels, but also indicate that a multi-faceted treatment approach could more effectively arrest tau accumulation in disease.

Hundreds of new mutant mouse lines are being produced annually using gene targeting and gene trap approaches in embryonic stem (ES) cells, and the number is expected to continue to grow as the human and mouse genome projects progress. The availability of robust ES cell lines and a simple technology for making chimeras is more attractive now than ever before. We established several new ES cell lines from 129/SvEv and C57BL/6 mice and tested their ability to contribute to the germline following blastocyst injections and/or the less expensive and easier method of morula-ES cell aggregation. Using morula aggregation to produce chimeras, five newly derived 129/SvEv and two C57BL/6 ES cell lines tested at early passages were found to contribute extensively to chimeras and produce germline-transmitting male chimeras. Furthermore, the two 129S/vEv ES cell lines that were tested and one of the C57BL/6 ES cell lines were able to maintain these characteristics after many passages in vitro. Our results indicate that the ability of ES cells to contribute strongly to chimeras following aggregation with outbred embryos is a general property of early passage ES cells and can be maintained for many passages. C56BL/6-derived ES cell lines, however, have a greater tendency than 129-derived ES cell lines to lose their ability to colonize the germline.