Nodal signals pattern vertebrate embryos

Vertebrate embryonic patterning requires several conserved inductive signals–including Nodal, Bmp, Wnt and Fgf signals. Nodal, which is a member of the transforming growth factor β (TGFβ) superfamily, activates a signal transduction pathway that is similar to that of other TGFβ members. Nodal genes, which have been identified in numerous vertebrate species, are expressed in specific cell types and tissues during embryonic development. Nodal signal transduction has been shown to play a pivotal role in inducing and patterning mesoderm and endoderm, and in regulating neurogenesis and left-right axis asymmetry. Antagonists, which act at different steps in the Nodal signal transduction pathway, have been shown to tightly modulate the inductive activity of Nodal. Received 20 October 2005; received after revision 15 November 2005; accepted 25 November 2005     

Expansion of amino acid homo-sequences in proteins: Insights into the role of amino acid homo-polymers and of the protein context in aggregation

Expansion of amino acid homo-sequences, such as polyglutamines or polyalanines, in proteins has been directly implicated in various degenerative diseases through a mechanism of protein misfolding and aggregation. However, it is still unclear how the nature of the expansion and the protein context influence the tendency of a protein to aggregate. Here, we have addressed these questions using spinocerebellar ataxia type-3 (ATX3) protein, the best characterised of the polyglutamine proteins, chosen as a model system. Using a transfected mammalian cell line, we demonstrate that ATX3 aggregation is noticeably reduced by deletion or replacement of regions other than the polyglutamine tract. The nature of the amino acid homo-sequences also has a strong influence on aggregation. From our studies, we draw general conclusions on the effect of the protein architecture and of the amino acid homo-sequence on pathology. Received 3 March 2006; received after revision 19 April 2006; accepted 22 May 2006     

Betaine homocysteine S-methyltransferase: just a regulator of homocysteine metabolism?

Betaine homocysteine methyltransferase (BHMT), a Zn²⁺-dependent thiolmethyltransferase, contributes to the regulation of homocysteine levels, increases in which are considered a risk factor for cardiovascular diseases. Most plasma homocysteine is generated through the liver methionine cycle, in which BHMT metabolizes approximately 25% of this non-protein amino acid. This process allows recovery of one of the three methylation equivalents used in phosphatidylcholine synthesis through transmethylation, a major homocysteine-producing pathway. Although BHMT has been known for over 40 years, the difficulties encountered in its isolation precluded detailed studies until very recently. Thus, the last 10 years, since the sequence became available, have yielded extensive structural and functional data. Moreover, recent findings offer clues for potential new functions for BHMT. The purpose of this review is to provide an integrated view of the knowledge available on BHMT, and to analyze its putative roles in other processes through interactions uncover to date. Received 26 May 2006; received after revision 3 July 2006; accepted 24 August 2006     

The ubiquitin-specific protease USP25 interacts with three sarcomeric proteins

The biological functions of the more than one hundred genes coding for deubiquitinating enzymes in the human genome remain mostly unknown. The USP25 gene, located at 21q11.2, encodes three protein isoforms produced by alternative splicing. While two of the isoforms are expressed nearly ubiquituously, the expression of the longer USP25 isoform (USP25m) is restricted to muscular tissues and is upregulated during myogenesis. USP25m interacts with three sarcomeric proteins: actin alpha-1 (ACTA1), filamin C (FLNC), and myosin binding protein C1 (MyBPC1), which are critically involved in muscle differentiation and maintenance, and have been implicated in the pathogenesis of severe myopathies. Biochemical analyses demonstrated that MyBPC1 is a short-lived proteasomal substrate, and its degradation is prevented by over-expression of USP25m but not by other USP25 isoforms. In contrast, ACTA1 and FLNC appear to be stable proteins, indicating that their interaction with USP25m is not related to their turnover rate. Received 7 November 2005; received after revision 7 January 2006; accepted 13 January 2006     

Liver X receptors in cardiovascular and metabolic disease

Liver X receptors (LXRs) α and β are nuclear oxysterol receptors and metabolic sensors initially found to regulate cholesterol metabolism and lipid biosynthesis. Recent studies have elucidated the importance of LXR in the development of cardiovascular diseases and metabolic disorders. LXR agonists prevent development of atherosclerosis by modulation of metabolic as well as inflammatory gene expression in rodent models. Moreover, LXR activation inhibits hepatic gluconeogenesis and lowers serum glucose levels, indicating possible application of LXR activation in the treatment of diabetes mellitus. However, first-generation LXR agonists elevate hepatic and serum trigylceride levels, making subtype-specific agonists and selective LXR modulators rather than unselective LXR agonists a potential pharmacological strategy. This review summarizes the multiple physiological and pathophysiological implications of LXRs and observations that identify LXRs as potential targets for therapeutic interventions in human cardiovascular and metabolic disease. Received 30 August 2005; received after revision 10 October 2005; accepted 4 November 2005     

Natural sweet macromolecules: how sweet proteins work

A few proteins, discovered mainly in tropical fruits, have a distinct sweet taste. These proteins have played an important role towards a molecular understanding of the mechanisms of taste. Owing to the huge difference in size, between most sweeteners and sweet proteins, it was believed that they must interact with a different receptor from that of small molecular weight sweeteners. Recent modelling studies have shown that the single sweet taste receptor has multiple active sites and that the mechanism of interaction of sweet proteins is intrinsically different from that of small sweeteners. Small molecular weight sweeteners occupy small receptor cavities inside two subdomains of the receptor, whereas sweet proteins can interact with the sweet receptor according to a mechanism called the ‘wedge model’ in which they bind to a large external cavity. This review describes these mechanisms and outlines a history of sweet proteins. Received 11 February 2006; received after revision 31 March 2006; accepted 11 May 2006     

Joint analysis is more efficient than replication-based analysis for two-stage genome-wide association studies

Genome-wide association is a promising approach to identify common genetic variants that predispose to human disease. Because of the high cost of genotyping hundreds of thousands of markers on thousands of subjects, genome-wide association studies often follow a staged design in which a proportion (pi(samples)) of the available samples are genotyped on a large number of markers in stage 1, and a proportion (pi(samples)) of these markers are later followed up by genotyping them on the remaining samples in stage 2. The standard strategy for analyzing such two-stage data is to view stage 2 as a replication study and focus on findings that reach statistical significance when stage 2 data are considered alone. We demonstrate that the alternative strategy of jointly analyzing the data from both stages almost always results in increased power to detect genetic association, despite the need to use more stringent significance levels, even when effect sizes differ between the two stages. We recommend joint analysis for all two-stage genome-wide association studies, especially when a relatively large proportion of the samples are genotyped in stage 1 (pi(samples) >or= 0.30), and a relatively large proportion of markers are selected for follow-up in stage 2 (pi(markers) >or= 0.01).     

Trak1 mutation disrupts GABA(A) receptor homeostasis in hypertonic mice

Hypertonia, which results from motor pathway defects in the central nervous system (CNS), is observed in numerous neurological conditions, including cerebral palsy, stroke, spinal cord injury, stiff-person syndrome, spastic paraplegia, dystonia and Parkinson disease. Mice with mutation in the hypertonic (hyrt) gene exhibit severe hypertonia as their primary symptom. Here we show that hyrt mutant mice have much lower levels of gamma-aminobutyric acid type A (GABA(A)) receptors in their CNS, particularly the lower motor neurons, than do wild-type mice, indicating that the hypertonicity of the mutants is likely to be caused by deficits in GABA-mediated motor neuron inhibition. We cloned the responsible gene, trafficking protein, kinesin binding 1 (Trak1), and showed that its protein product interacts with GABA(A) receptors. Our data implicate Trak1 as a crucial regulator of GABA(A) receptor homeostasis and underscore the importance of hyrt mice as a model for studying the molecular etiology of hypertonia associated with human neurological diseases.     

Common loss-of-function variants of the epidermal barrier protein filaggrin are a major predisposing factor for atopic dermatitis

Atopic disease, including atopic dermatitis (eczema), allergy and asthma, has increased in frequency in recent decades and now affects approximately 20% of the population in the developed world. Twin and family studies have shown that predisposition to atopic disease is highly heritable. Although most genetic studies have focused on immunological mechanisms, a primary epithelial barrier defect has been anticipated. Filaggrin is a key protein that facilitates terminal differentiation of the epidermis and formation of the skin barrier. Here we show that two independent loss-of-function genetic variants (R510X and 2282del4) in the gene encoding filaggrin (FLG) are very strong predisposing factors for atopic dermatitis. These variants are carried by approximately 9% of people of European origin. These variants also show highly significant association with asthma occurring in the context of atopic dermatitis. This work establishes a key role for impaired skin barrier function in the development of atopic disease.