Nspl1, a new Z-band-associated protein

Kindlin-2 interacts with α-actinin-2 and β1 integrin to maintain the integrity of the Z-disc in cardiac muscles

Kindlin-2, as an integrin-interacting protein, was known to be required for the maintenance of cardiac structure and function in zebrafish. However, the mechanism remains unclear. We found that Kindlin-2 interacts and colocalizes with α-actinin-2 at the Z-disc of mouse cardiac muscles and there Kindlin-2 also interacts with β1 integrin. Knockdown of Kindlin-2 influences the association of β1 integrin with α-actinin-2 and disrupts the structure of the Z-disc and leads to cardiac dysfunction. Our data indicated that Kindlin-2 is a novel α-actinin-2-interacting protein and plays an important role in the regulation of cardiac structure and function.

Functional role of neuroendocrine-specific protein-like 1 in membrane translocation of GLUT4

OBJECTIVE

In skeletal muscles, dantrolene inhibits the exercise-induced membrane translocation of GLUT4. It has been postulated that the inhibitory action of dantrolene on Ca(2+) release from the sarcoplasmic reticulum (SR) causes inhibition of exercise-induced glucose uptake; however, the precise mechanism has not been adequately studied.

RESEARCH DESIGN AND METHODS

We discovered that dantrolene can bind to skeletal-type neuroendocrine-specific protein-like 1 (sk-NSPl1) with photoreactive dantrolene derivatives. In sk-NSPl1-deficient muscles, we examined the change in glucose uptake and the membrane translocation of GLUT4. In addition, we examined the change in blood glucose and also measured the glycogen level in both isolated and in situ skeletal muscles after electrical stimulation using our mutant mouse.

RESULTS

In sk-NSPl1-deficient muscles, exercise-induced glucose uptake was totally abolished with no change in insulin-induced glucose uptake. The Ca(2+) release mechanism and its inhibition by dantrolene were completely preserved in these muscles. The expression of GLUT4 in the mutant muscles also appeared unchanged. Confocal imaging and results using the membrane isolation method showed that exercise/contraction did not enhance GLUT4 translocation in these sk-NSPl1-deficient muscles under conditions of adequate muscle glycogen consumption. The blood glucose level in normal mice was reduced by electrical stimulation of the hind limbs, but that in mutant mice was unchanged.

CONCLUSIONS

sk-NSPl1 is a novel dantrolene receptor that plays an important role in membrane translocation of GLUT4 induced by contraction/exercise. The 23-kDa sk-NSPl1 may also be involved in the regulation of glucose levels in the whole body.

Reticulons as markers of neurological diseases: focus on amyotrophic lateral sclerosis

Reticulons (RTNs) are a family of proteins that are primarily associated with the endoplasmic reticulum. In mammals, four genes have been identified and referred as to rtn1, 2, 3 and the neurite outgrowth inhibitor rtn4/nogo. These genes generate multiple isoforms that contain a common C-terminal reticulon homology domain of 150-200 amino-acid residues. The N-terminal regions of RTNs are highly variable, and result from alternative splicing or differential promoter usage. Although widely distributed, the functions of RTNs are still poorly understood. Much interest has been focused on rtn4/nogo because of its activity as a potent inhibitor of axonal growth and repair. In the present study, we update recent knowledge on mammalian RTNs paying special attention to the involvement of these proteins as markers of neurological diseases. We also present recent data concerning RTN expression in amyotrophic lateral sclerosis, a fatal degenerative disorder characterized by loss of upper and lower motor neurons, and muscle atrophy. The rearrangement of RTN expression is regulated not only in suffering skeletal muscle but also preceding the onset of symptoms, and may relate to the disease process.

Tissue specificity and regulation of the N-terminal diversity of reticulon 3

Over the last few years, the widely distributed family of reticulons (RTNs) is receiving renewed interest because of the implication of RTN4/Nogo in neurite regeneration. Four genes were identified in mammals and are referred to as RTN1, 2, 3 and the neurite outgrowth inhibitor RTN4/Nogo. In the present paper, we describe the existence of five new isoforms of RTN3 that differ in their N-termini, and analysed their tissue distribution and expression in neurons. We redefined the structure of human and murine rtn3 genes, and identified two supplementary exons that may generate up to seven putative isoforms arising by alternative splicing or differential promoter usage. We confirmed the presence of five of these isoforms at the mRNA and protein levels, and showed their preferential expression in the central nervous system. We analysed rtn3 expression in the cerebellum further, and observed increased levels of several of the RTN3 isoforms during cerebellum development and during in vitro maturation of cerebellar granule cells. This pattern of expression paralleled that shown by RTN4/Nogo isoforms. Specifically, RTN3A1 expression was down-regulated upon cell death of cerebellar granule neurons triggered by potassium deprivation. Altogether, our results demonstrate that the rtn3 gene generates multiple isoforms varying in their N-termini, and that their expression is tightly regulated in neurons. These findings suggest that RTN3 isoforms may contribute, by as yet unknown mechanisms, to neuronal survival and plasticity.

Nogo and its paRTNers

Reticulons (RTNs) are a relatively new eukaryotic gene family with unknown functions but broad expression and peculiar topological features. RTNs are widely distributed in plants, yeast and animals and are characterized by a approximately 200-amino-acid C-terminal domain, including two long hydrophobic sequences. Nogo/RTN4 can inhibit neurite growth from the cell surface via specific receptors, whereas more general, 'ancestral', RTN functions might relate to those of the endoplasmic reticulum - for example, intracellular trafficking, cell division and apoptosis. Here, we review the taxonomic distribution and tissue expression of RTNs, summarize recent discoveries about RTN localization and membrane topology, and discuss the possible functions of RTNs.

Genomic structure and functional characterisation of the promoters of human and mouse nogo/rtn4

The reticulon-family member Nogo-A is a potent neurite growth inhibitory protein in vitro and may play a role in the restriction of axonal regeneration after injury and of structural plasticity in the CNS of higher vertebrates. Of the three major isoforms of Nogo, Nogo-A is mostly expressed in the brain, Nogo-B is found in a ubiquitous pattern, and Nogo-C is most highly expressed in muscle. Seven additional splice-variants derived both from differential splicing and differential promoter usage have been identified. Analysis of the TATA-less Nogo-A/B promoter (P1) shows that conserved GC-boxes and a CCAAT-box within the first 500bp upstream of the transcription start are responsible for its regulation. No major differences in the methylation status of the P1 CpG-island in tissues expressing or not expressing Nogo-A/B could be detected, suggesting that silencer elements are involved in the regulation. The specific expression pattern of Nogo-A/B is due to differential splicing. The basal Nogo-C promoter (P2) is regulated by a proximal and a distal element. The 5'UTR of Nogo-C harbours a negative control element. These data may help to identify factors that can modulate Nogo transcription, thus offering an alternative approach for Nogo neutralisation.

The distribution of cofilin and DNase I in vivo

Actin is the principal component of the cytoskeleton, a structure that can be disassembled and reassembled in a matter of seconds in vivo. The state of assembly of actin in vivo is primarily regulated by one or more actin binding proteins (ABPs). Typically, the actions of ABPs have been studied one by one, however, we propose that multiple ABPs, acting cooperatively, may be involved in the control of actin filament length. Cofilin and DNase I are two ABPs that have previously been demonstrated to form a ternary complex with actin in vitro. This is the first report to demonstrate their co-localisation in vivo, and differences in their distributions. Our observations strongly suggest a physiological role for higher order complexes of actin in regulation of cytoskeletal assembly during processes such as cell division.

Fishing out proteins that bind to titin

Another giant protein has been detected in cross-striated muscle cells. Given the name obscurin, it was discovered in a yeast two-hybrid screen in which the bait was a small region of titin that is localized near the Z-band. Obscurin is about 720 kD, similar in molecular weight to nebulin, but present at about one tenth the level (Young et al., 2001). Like titin, obscurin contains multiple immunoglobulin-like domains linked in tandem, but in contrast to titin it contains just two fibronectin-like domains. It also contains sequences that suggest obscurin may have roles in signal transduction. During embryonic development, its localization changes from the Z-band to the M-band. With these intriguing properties, obscurin may not remain obscure for long.

Myozenin: An -actinin- and -filamin-binding protein of skeletal muscle Z lines

To better understand the structure and function of Z lines, we used sarcomeric isoforms of alpha-actinin and gamma-filamin to screen a human skeletal muscle cDNA library for interacting proteins by using the yeast two-hybrid system. Here we describe myozenin (MYOZ), an alpha-actinin- and gamma-filamin-binding Z line protein expressed predominantly in skeletal muscle. Myozenin is predicted to be a 32-kDa, globular protein with a central glycine-rich domain flanked by alpha-helical regions with no strong homologies to any known genes. The MYOZ gene has six exons and maps to human chromosome 10q22.1-q22.2. Northern blot analysis demonstrated that this transcript is expressed primarily in skeletal muscle with significantly lower levels of expression in several other tissues. Antimyozenin antisera stain skeletal muscle in a sarcomeric pattern indistinguishable from that seen by using antibodies for alpha-actinin, and immunogold electron microscopy confirms localization specifically to Z lines. Thus, myozenin is a skeletal muscle Z line protein that may be a good candidate gene for limb-girdle muscular dystrophy or other neuromuscular disorders.

Myozenin: an alpha-actinin- and gamma-filamin-binding protein of skeletal muscle Z lines

To better understand the structure and function of Z lines, we used sarcomeric isoforms of alpha-actinin and gamma-filamin to screen a human skeletal muscle cDNA library for interacting proteins by using the yeast two-hybrid system. Here we describe myozenin (MYOZ), an alpha-actinin- and gamma-filamin-binding Z line protein expressed predominantly in skeletal muscle. Myozenin is predicted to be a 32-kDa, globular protein with a central glycine-rich domain flanked by alpha-helical regions with no strong homologies to any known genes. The MYOZ gene has six exons and maps to human chromosome 10q22.1-q22.2. Northern blot analysis demonstrated that this transcript is expressed primarily in skeletal muscle with significantly lower levels of expression in several other tissues. Antimyozenin antisera stain skeletal muscle in a sarcomeric pattern indistinguishable from that seen by using antibodies for alpha-actinin, and immunogold electron microscopy confirms localization specifically to Z lines. Thus, myozenin is a skeletal muscle Z line protein that may be a good candidate gene for limb-girdle muscular dystrophy or other neuromuscular disorders.

alpha B-crystallin gene induction and phosphorylation by MKK6-activated p38. A potential role for alpha B-crystallin as a target of the p38 branch of the cardiac stress response

The MAPK kinase MKK6 selectively stimulates p38 MAPK and confers protection against stress-induced apoptosis in cardiac myocytes. However, the events lying downstream of p38 that mediate this protection are unknown. The small heat shock protein, alphaB-crystallin, which is expressed in only a few cell types, including cardiac myocytes, may participate in MKK6-mediated cytoprotection. In the present study, we showed that, in cultured cardiac myocytes, expression of MKK6(Glu), an active form of MKK6, led to p38-dependent increases in alphaB-crystallin mRNA, protein, and transcription. MKK6(Glu) also induced p38-dependent activation of the downstream MAPK-activated protein kinase, MAPKAP-K2, and the phosphorylation of alphaB-crystallin on serine-59. Initially, exposure of cells to the hyperosmotic stressor, sorbitol, stimulated MKK6, p38, and MAPKAP-K2 and increased phosphorylation of alphaB-crystallin on serine 59. However, after longer times of exposure to sorbitol, the cells began to undergo apoptosis. This sorbitol-induced apoptosis was increased when p38 was inhibited in a manner that would block alphaB-crystallin induction and phosphorylation. Thus, under these conditions, the activation of MKK6, p38, and MAPKAP-K2 by sorbitol can provide a degree of protection against stress-induced apoptosis. Supporting this view was the finding that sorbitol-induced apoptosis was nearly completely blocked in cells expressing MKK6(Glu). Therefore, the cytoprotective effects of MKK6 in cardiac myocytes are due, in part, to phosphorylation of alphaB-crystallin on serine 59 and to the induction of alphaB-crystallin gene expression.