Platelet adhesion: Structural and functional diversity of short dystrophin and utrophins in the formation of dystrophinassociated-protein complexes related to actin dynamics

Platelets are dynamic cell fragments that modify their shape during activation. Utrophin and dystrophins are minor actin-binding proteins present in muscle and non-muscle cytoskeleton. In the present study, we characterised the pattern of Dp71 isoforms and utrophin gene products by immunoblot in human platelets. Two new dystrophin isoforms were found, Dp71f and Dp71d, as well as the Up71 isoform and the dystrophin-associated proteins, α and β-dystrobrevins. Distribution of Dp71d/Dp71Δ110 m, Up400/Up71 and dystrophin-associated proteins in relation to the actin cytoskeleton was evaluated by confocal microscopy in both resting and platelets adhered on glass. Formation of two dystrophin-associated protein complexes (Dp71d/Dp71Δ110 m ~DAPC and Up400/Up71~DAPC) was demonstrated by co-immunoprecipitation and their distribution in relation to the actin cytoskeleton was characterised during platelet adhesion. The Dp71d/Dp71Δ110 m ~DAPC is maintained mainly at the granulomere and is associated with dynamic structures during activation by adhesion to thrombin-coated surfaces. Participation of both Dp71d/Dp71Δ110 m ~DAPC and Up400/Up71~DAPC in the biological roles of the platelets is discussed.

E2F transcription factor-1 deficiency reduces pathophysiology in the mouse model of Duchenne muscular dystrophy through increased muscle oxidative metabolism

E2F1 deletion leads to increased mitochondrial number and function, increased body temperature in response to cold and increased resistance to fatigue with exercise. Since E2f1−/− mice show increased muscle performance, we examined the effect of E2f1 genetic inactivation in the mdx background, a mouse model of Duchenne muscular dystrophy (DMD). E2f1−/−;mdx mice demonstrated a strong reduction of physiopathological signs of DMD, including preservation of muscle structure, decreased inflammatory profile, increased utrophin expression, resulting in better endurance and muscle contractile parameters, comparable to normal mdx mice. E2f1 deficiency in the mdx genetic background increased the oxidative metabolic gene program, mitochondrial activity and improved muscle functions. Interestingly, we observed increased E2F1 protein levels in DMD patients, suggesting that E2F1 might represent a promising target for the treatment of DMD.

Upregulation of brain utrophin does not rescue behavioral alterations in dystrophin-deficient mice

Dystrophin, the protein responsible for X-linked Duchenne muscular dystrophy (DMD), is normally expressed in both muscle and brain, which explains that its loss also leads to cognitive deficits. The utrophin protein, an autosomal homolog, is a natural candidate for dystrophin replacement in patients. Pharmacological upregulation of endogenous utrophin improves muscle physiology in dystrophin-deficient mdx mice, and represents a potential therapeutic tool that has the advantage of allowing delivery to various organs following peripheral injections. Whether this could alleviate cognitive deficits, however, has not been explored. Here, we first investigated basal expression of all utrophins and dystrophins in the brain of mdx mice and found no evidence for spontaneous compensation by utrophins. Then, we show that systemic chronic, spaced injections of arginine butyrate (AB) alleviate muscle alterations and upregulate utrophin expression in the adult brain of mdx mice. AB selectively upregulated brain utrophin Up395, while reducing expression of Up113 and Up71. This, however, was not associated with a significant improvement of behavioral functions typically affected in mdx mice, which include exploration, emotional reactivity, spatial and fear memories. We suggest that AB did not overcome behavioral and cognitive dysfunctions because the regional and cellular expression of utrophins did not coincide with dystrophin expression in untreated mice, nor did it in AB-treated mice. While treatments based on the modulation of utrophin may alleviate DMD phenotypes in certain organs and tissues that coexpress dystrophins and utrophins in the same cells, improvement of cognitive functions would likely require acting on specific dystrophin-dependent mechanisms.

In vitro mouse model in Duchenne muscular dystrophy diagnosis using 50-MHz ultrasound waves

Duchenne muscular dystrophy (DMD) is caused by the absence of dystrophin, the protein that plays a key mechanical role in maintaining muscle membrane integrity. One of the major consequences of dystrophin deficiency is the degeneration of muscle fibres, with a progressive loss in muscle strength. The objective of this research was to find an ultrasonic parameter sensitive to DMD, which could give relevant information related to microstructure if compared to traditional investigations such as morphometrical analysis. This "in vitro" study focused on the Mdx mouse model and investigated the potential differences between wild-type and dystrophin-deficient mice diaphragms. Using a 50MHz ultrasonic sensor built in our group, we recorded an increase in ultrasonic wave attenuation in the dystrophin-deficient samples in comparison with normal muscles. A correlation between attenuation, mouse age and the percentage of non-muscular proportion in muscle was observed. As Mdx mouse is the best animal model for DMD and reproduces the degenerative pattern observed in human DMD muscles, this approach could be a powerful tool for in vitro DMD investigation and, more generally, for the characterisation of muscle properties.

Functional implication of Dp71 in osmoregulation and vascular permeability of the retina

Functional alterations of Müller cells, the principal glia of the retina, are an early hallmark of most retina diseases and contribute to their further progression. The molecular mechanisms of these reactive Müller cell alterations, resulting in disturbed retinal homeostasis, remain largely unknown. Here we show that experimental detachment of mouse retina induces mislocation of the inwardly rectifying potassium channels (Kir4.1) and a downregulation of the water channel protein (AQP4) in Müller cells. These alterations are associated with a strong decrease of Dp71, a cytoskeleton protein responsible for the localization and the clustering of Kir4.1 and AQP4. Partial (in detached retinas) or total depletion of Dp71 in Müller cells (in Dp71-null mice) impairs the capability of volume regulation of Müller cells under osmotic stress. The abnormal swelling of Müller cells In Dp71-null mice involves the action of inflammatory mediators. Moreover, we investigated whether the alterations in Müller cells of Dp71-null mice may interfere with their regulatory effect on the blood-retina barrier. In the absence of Dp71, the retinal vascular permeability was increased as compared to the controls. Our results reveal that Dp71 is crucially implicated in the maintenance of potassium homeostasis, in transmembraneous water transport, and in the Müller cell-mediated regulation of retinal vascular permeability. Furthermore, our data provide novel insights into the mechanisms of retinal homeostasis provided by Müller cells under normal and pathological conditions.

Dystrophin Dp71f associates with components of the beta1-integrin adhesion complex in PC12 cell neurites

Dystrophin Dp71 has been implicated with cognitive impairment shown by Duchenne muscular dystrophy patients. To study Dp71 neural role, we used PC12 cell line, since these cells differentiate into sympathetic like neurons when stimulated with nerve growth factor Previously in undiferentiated PC12 cells, it was demonstrated that dystrophin Dp71f is a key component of the beta1-integrin adhesion complex that confers proper complex assembly. Since integrin based mediated adhesion is important during neuronal differentiation, it was important to know if dystrophin Dp71f was a structural component of the beta1-integrin adhesion complex in neurites of nerve growth factor stimulated PC12 cells. In the present work, by performing immunofluorescence assays, we determined the association of dystrophin Dp71f with some components of the beta1-integrin adhesion complex such as beta1-integrin subunit, talin, alpha-actinin and vinculin in neurites of nerve growth factor stimulated PC12 cells seeded onto the extracellular matrix protein laminin. The association was stronger in neural growth cones suggesting that dystrophin Dp71f is important for the function that the beta1-integrin complex has during neurite outgrowth.

Nuclear and nuclear envelope localization of dystrophin Dp71 and dystrophin-associated proteins (DAPs) in the C2C12 muscle cells: DAPs nuclear localization is modulated during myogenesis

Dystrophin and dystrophin-associated proteins (DAPs) form a complex around the sarcolemma, which gives stability to the sarcolemma and leads signal transduction. Recently, the nuclear presence of dystrophin Dp71 and DAPs has been revealed in different non-muscle cell types, opening the possibility that these proteins could also be present in the nucleus of muscle cells. In this study, we analyzed by Immunofluorescence assays and Immunoblotting analysis of cell fractions the subcellular localization of Dp71 and DAPs in the C(2)C(12) muscle cell line. We demonstrated the presence of Dp71, alpha-sarcoglycan, alpha-dystrobrevin, beta-dystroglycan and alpha-syntrophin not only in plasma membrane but also in the nucleus of muscle cells. In addition, we found by Immunoprecipitation assays that these proteins form a nuclear complex. Interestingly, myogenesis modulates the presence and/or relative abundance of DAPs in the plasma membrane and nucleus as well as the composition of the nuclear complex. Finally, we demonstrated the presence of Dp71, alpha-sarcoglycan, beta-dystroglycan, alpha-dystrobrevin and alpha-syntrophin in the C(2)C(12) nuclear envelope fraction. Interestingly, alpha-sarcoglycan and beta-dystroglycan proteins showed enrichment in the nuclear envelope, compared with the nuclear fraction, suggesting that they could function as inner nuclear membrane proteins underlying the secondary association of Dp71 and the remaining DAPs to the nuclear envelope. Nuclear envelope localization of Dp71 and DAPs might be involved in the nuclear envelope-associated functions, such as nuclear structure and modulation of nuclear processes.

Biochemical properties of gastrokine-1 purified from chicken gizzard smooth muscle

The potential role and function of gastrokine-1 (GNK1) in smooth muscle cells is investigated in this work by first establishing a preparative protocol to obtain this native protein from freshly dissected chicken gizzard. Some unexpected biochemical properties of gastrokine-1 were deduced by producing specific polyclonal antibody against the purified protein. We focused on the F-actin interaction with gastrokine-1 and the potential role and function in smooth muscle contractile properties.

Background

GNK1 is thought to provide mucosal protection in the superficial gastric epithelium. However, the actual role of gastrokine-1 with regards to its known decreased expression in gastric cancer is still unknown. Recently, trefoil factors (TFF) were reported to have important roles in gastric epithelial regeneration and cell turnover, and could be involved in GNK1 interactions. The aim of this study was to evaluate the role and function of GNK1 in smooth muscle cells.

Methodology/Principal Findings

From fresh chicken gizzard smooth muscle, an original purification procedure was used to purify a heat soluble 20 kDa protein that was sequenced and found to correspond to the gastrokine-1 protein sequence containing one BRICHOS domain and at least two or possibly three transmembrane regions. The purified protein was used to produce polyclonal antibody and highlighted the smooth muscle cell distribution and F-actin association of GNK1 through a few different methods.

Conclusion/Significance

Altogether our data illustrate a broader distribution of gastrokine-1 in smooth muscle than only in the gastrointestinal epithelium, and the specific interaction with F-actin highlights and suggests a new role and function of GNK1 within smooth muscle cells. A potential role via TFF interaction in cell-cell adhesion and assembly of actin stress fibres is discussed.

L-arginine decreases inflammation and modulates the nuclear factor-kappaB/matrix metalloproteinase cascade in mdx muscle fibers

Duchenne muscular dystrophy (DMD) is a lethal, X-linked disorder associated with dystrophin deficiency that results in chronic inflammation, sarcolemma damage, and severe skeletal muscle degeneration. Recently, the use of L-arginine, the substrate of nitric oxide synthase (nNOS), has been proposed as a pharmacological treatment to attenuate the dystrophic pattern of DMD. However, little is known about signaling events that occur in dystrophic muscle with l-arginine treatment. Considering the implication of inflammation in dystrophic processes, we asked whether L-arginine inhibits inflammatory signaling cascades. We demonstrate that L-arginine decreases inflammation and enhances muscle regeneration in the mdx mouse model. Classic stimulatory signals, such as proinflammatory cytokines interleukin-1beta, interleukin-6, and tumor necrosis factor-alpha, are significantly decreased in mdx mouse muscle, resulting in lower nuclear factor (NF)-kappaB levels and activity. NF-kappaB serves as a pivotal transcription factor with multiple levels of regulation; previous studies have shown perturbation of NF-kappaB signaling in both mdx and DMD muscle. Moreover, L-arginine decreases the activity of metalloproteinase (MMP)-2 and MMP-9, which are transcriptionally activated by NF-kappaB. We show that the inhibitory effect of L-arginine on the NF-kappaB/MMP cascade reduces beta-dystroglycan cleavage and translocates utrophin and nNOS throughout the sarcolemma. Collectively, our results clarify the molecular events by which L-arginine promotes muscle membrane integrity in dystrophic muscle and suggest that NF-kappaB-related signaling cascades could be potential therapeutic targets for DMD management.

Muscle dystroglycan organizes the postsynapse and regulates presynaptic neurotransmitter release at the Drosophila neuromuscular junction

Background

The Dystrophin-glycoprotein complex (DGC) comprises dystrophin, dystroglycan, sarcoglycan, dystrobrevin and syntrophin subunits. In muscle fibers, it is thought to provide an essential mechanical link between the intracellular cytoskeleton and the extracellular matrix and to protect the sarcolemma during muscle contraction. Mutations affecting the DGC cause muscular dystrophies. Most members of the DGC are also concentrated at the neuromuscular junction (NMJ), where their deficiency is often associated with NMJ structural defects. Hence, synaptic dysfunction may also intervene in the pathology of dystrophic muscles. Dystroglycan is a central component of the DGC because it establishes a link between the extracellular matrix and Dystrophin. In this study, we focused on the synaptic role of Dystroglycan (Dg) in Drosophila.

Methodology/Principal Findings

We show that Dg was concentrated postsynaptically at the glutamatergic NMJ, where, like in vertebrates, it controls the concentration of synaptic Laminin and Dystrophin homologues. We also found that synaptic Dg controlled the amount of postsynaptic 4.1 protein Coracle and alpha-Spectrin, as well as the relative subunit composition of glutamate receptors. In addition, both Dystrophin and Coracle were required for normal Dg concentration at the synapse. In electrophysiological recordings, loss of postsynaptic Dg did not affect postsynaptic response, but, surprisingly, led to a decrease in glutamate release from the presynaptic site.

Conclusion/Significance

Altogether, our study illustrates a conservation of DGC composition and interactions between Drosophila and vertebrates at the synapse, highlights new proteins associated with this complex and suggests an unsuspected trans-synaptic function of Dg.

Ventilation during air breathing and in response to hypercapnia in 5 and 16 month-old mdx and C57 mice

Previous studies have shown a blunted ventilatory response to hypercapnia in mdx mice older than 7 months. We test the hypothesis that in the mdx mice ventilatory response changes with age, concomitantly with the increased functional impairment of the respiratory muscles. We thus studied the ventilatory response to CO(2) in 5 and 16 month-old mdx and C57BL10 mice (n = 8 for each group). Respiratory rate (RR), tidal volume (VT), and minute ventilation (VE) were measured, using whole-body plethysmography, during air breathing and in response to hypercapnia (3, 5 and 8% CO(2)). The ventilatory protocol was completed by histological analysis of the diaphragm and intercostals muscles. During air breathing, the 16 month-old mdx mice showed higher RR and, during hypercapnia (at 8% CO(2) breathing), significantly lower RR (226 +/- 26 vs. 270 +/- 21 breaths/min) and VE (1.81 +/- 0.35 vs. 3.96 +/- 0.59 ml min(-1) g(-1)) (P < 0.001) in comparison to C57BL10 controls. On the other hand, 5 month-old C57BL10 and mdx mice did not present any difference in their ventilatory response to air breathing and to hypercapnia. In conclusion, this study shows similar ventilation during air breathing and in response to hypercapnia in the 5 month-old mdx and control mice, in spite of significant pathological structural changes in the respiratory muscles of the mdx mice. However in the 16 month-old mdx mice we observed altered ventilation under air and blunted ventilation response to hypercapnia compared to age-matched control mice. Ventilatory response to hypercapnia thus changes with age in mdx mice, in line with the increased histological damage of their respiratory muscles.

Modulation of p38 mitogen-activated protein kinase cascade and metalloproteinase activity in diaphragm muscle in response to free radical scavenger administration in dystrophin-deficient Mdx mice

Duchenne muscular dystrophy muscles undergo increased oxidative stress and altered calcium homeostasis, which contribute to myofiber loss by trigging both necrosis and apoptosis. Here, we asked whether treatment with free radical scavengers could improve the dystrophic pattern of mdx muscles. Five-week-old mdx mice were treated for 2 weeks with alpha-lipoic acid/l-carnitine. This treatment decreased the plasmatic creatine kinase level, the antioxidant enzyme activity, and lipid peroxidation products in mdx diaphragm. Free radical scavengers also modulated the phosphorylation/activity of some component of the mitogen-activated protein kinase (MAPK) cascades: p38 MAPK, the extracellular signal-related kinase, and the Jun kinase. beta-Dystroglycan (beta-DG), a multifunctional adaptor or scaffold capable of interacting with components of the extracellular signal-related kinase-MAP kinase cascade, was also affected after treatment. In the mdx muscles, beta-DG (43 kd) was cleaved by matrix metalloproteinases into a 30-kd form (beta-DG30). We show that the proinflammatory protein nuclear factor-kappaB activator decreased after the treatment, leading to a significant reduction of matrix metalloproteinase activity in the mdx diaphragm. Our data highlight the implication of oxidative stress and cell signaling defects in dystrophin-deficient muscle via the MAP kinase cascade-beta-DG interaction and nuclear factor-kappaB-mediated inflammation process.

ZZ domain of dystrophin and utrophin: topology and mapping of a beta-dystroglycan interaction site

Dystrophin forms part of a vital link between actin cytoskeleton and extracellular matrix via the transmembrane adhesion receptor dystroglycan. Dystrophin and its autosomal homologue utrophin interact with beta-dystroglycan via their highly conserved C-terminal cysteine-rich regions, comprising the WW domain (protein-protein interaction domain containing two conserved tryptophan residues), EF hand and ZZ domains. The EF hand region stabilizes the WW domain providing the main interaction site between dystrophin or utrophin and dystroglycan. The ZZ domain, containing a predicted zinc finger motif, stabilizes the WW and EF hand domains and strengthens the overall interaction between dystrophin or utrophin and beta-dystroglycan. Using bacterially expressed ZZ domain, we demonstrate a conformational effect of zinc binding to the ZZ domain, and identify two zinc-binding regions within the ZZ domain by SPOTs overlay assays. Epitope mapping of the dystrophin ZZ domain was carried out with new monoclonal antibodies by ELISA, overlay assay and immunohistochemistry. One monoclonal antibody defined a discrete region of the ZZ domain that interacts with beta-dystroglycan. The epitope was localized to the conformationally sensitive second zinc-binding site in the ZZ domain. Our results suggest that residues 3326-3332 of dystrophin form a crucial part of the contact region between dystrophin and beta-dystroglycan and provide new insight into ZZ domain organization and function.

Dp71ab/DAPs complex composition changes during the differentiation process in PC12 cells

PC12 cells express different Dp71 isoforms originated from alternative splicing; one of them, Dp71ab lacks exons 71 and 78. To gain insight into the function of Dp71 isoforms we identified dystrophin associated proteins (DAPs) that associate in vivo with Dp71ab during nerve growth factor (NGF) induced differentiation of PC12 cells. DAPs expression was analyzed by RT-PCR, Western blot and indirect immunofluorescence, showing the presence of each mRNA and protein corresponding to alpha-, beta-, gamma-, delta-, and epsilon-sarcoglycans as well as zeta-sarcoglycan mRNA. Western blot analysis also revealed the expression of beta-dystroglycan, alpha1-syntrophin, alpha1-, and beta-dystrobrevins. We have established that Dp71ab forms a complex with beta-dystroglycan, alpha1-syntrophin, beta-dystrobrevin, and alpha-, beta- and gamma-sarcoglycans in undifferentiated PC12 cells. In differentiated PC12 cells, the complex composition changes since Dp71ab associates only with beta-dystroglycan, alpha1-syntrophin, beta-dystrobrevin, and delta-sarcoglycan. Interestingly, neuronal nitric oxide synthase associates with the Dp71ab/DAPs complex during NGF treatment, raising the possibility that Dp71ab may be involved in signal transduction events during neuronal differentiation.

Characterization of a novel Dp71 dystrophin-associated protein complex (DAPC) present in the nucleus of HeLa cells: members of the nuclear DAPC associate with the nuclear matrix

Dystrophin is an essential component in the assembly and maintenance of the dystrophin-associated protein complex (DAPC), which includes members of the dystroglycan, syntrophin, sarcoglycan and dystrobrevin protein families. Distinctive complexes have been described in the cell membrane of different tissues and cultured cells. In this work, we report the identification and characterization of a novel DAPC present in the nuclei of HeLa cells, which contains dystrophin Dp71 as a key component. Using confocal microscopy and cell fractionation analyses, we found the presence of Dp71, beta-sarcoglycan, beta-dystroglycan, alpha- and beta-syntrophin, alpha1- and beta-dystrobrevin and nNOS in the nuclei of HeLa cells. Furthermore, we demonstrated by co-immunoprecipitation experiments that most of these proteins form a complex in the nuclear compartment. Next, we analyze the possible association of the nuclear DAPC with the nuclear matrix. We found the presence of Dp71, beta-dystroglycan, nNOS, beta-sarcoglycan, alpha/beta syntrophin, alpha1-dystrobrevin and beta-dystrobrevin in the nuclear matrix protein fractions and in situ nuclear matrix preparations from HeLa cells. Moreover, we found that Dp71, beta-dystroglycan and beta-dystrobrevin co-immunoprecipitated with the nuclear matrix proteins lamin B1 and actin. The association of members of the nuclear DAPC with the nuclear matrix indicates that they may work as scaffolding proteins involved in nuclear architecture.

Dystrophin Dp71f associates with the beta1-integrin adhesion complex to modulate PC12 cell adhesion

Dystrophin Dp71 is the main product of the Duchenne muscular dystrophy gene in the brain; however, its function is unknown. To study the role of Dp71 in neuronal cells, we previously generated by antisense treatment PC12 neuronal cell clones with decreased Dp71 expression (antisense-Dp71 cells). PC12 cells express two different splicing isoforms of Dp71, a cytoplasmic variant called Dp71f and a nuclear isoform called Dp71d. We previously reported that antisense-Dp71 cells display deficient adhesion to substrate and reduced immunostaining of beta1-integrin in the cell area contacting the substrate. In this study, we isolated additional antisense-Dp71 clones to analyze in detail the potential involvement of Dp71f isoform with the beta1-integrin adhesion system of PC12 cells. Immunofluorescence analyses as well as immunoprecipitation assays demonstrated that the PC12 cell beta1-integrin adhesion complex is composed of beta1-integrin, talin, paxillin, alpha-actinin, FAK and actin. In addition, our results showed that Dp71f associates with most of the beta1-integrin complex components (beta1-integrin, FAK, alpha-actinin, talin and actin). In the antisense-Dp71 cells, the deficiency of Dp71 provokes a significant reduction of the beta1-integrin adhesion complex and, consequently, the deficient adhesion of these cells to laminin. In vitro binding experiments confirmed the interaction of Dp71f with FAK and beta1-integrin. Our data indicate that Dp71f is a structural component of the beta1-integrin adhesion complex of PC12 cells that modulates PC12 cell adhesion by conferring proper complex assembly and/or maintenance.

Effect of beta-dystroglycan processing on utrophin/Dp116 anchorage in normal and mdx mouse Schwann cell membrane

In the peripheral nervous system, utrophin and the short dystrophin isoform (Dp116) are co-localized at the outermost layer of the myelin sheath of nerve fibers; together with the dystroglycan complex. Dp116 is associated with multiple glycoproteins, i.e. sarcoglycans, and alpha- and beta-dystroglycan, which anchor the cytoplasmic protein subcomplex to the extracellular basal lamina. In peripheral nerve, matrix metalloproteinase activity disrupts the dystroglycan complex by cleaving the extracellular domain of beta-dystroglycan. Metalloproteinase creates a 30 kDa fragment of beta-dystroglycan, leading to a disruption of the link between the extracellular matrix and the cell membrane. Here we asked if the processing of the beta-dystroglycan could influence the anchorage of Dp116 and/or utrophin in normal and mdx Schwann cell membrane. We showed that metalloproteinase-9 was more activated in mdx nerve than in wild-type ones. This activation leads to an accumulation of the 30 kDa beta-dystroglycan isoform and has an impact on the anchorage of Dp116 and utrophin isoforms in mdx Schwann cells membrane. Our results showed that Dp116 had greater affinity to the full length form of beta-dystroglycan than the 30 kDa form. Moreover, we showed for the first time that the short isoform of utrophin (Up71) was over-expressed in mdx Schwann cells compared with wild-type. In addition, this utrophin isoform (Up71) seems to have greater affinity to the 30 kDa beta-dystroglycan which could explain the increased stabilization of this 30 kDa form at the membrane compartment. Our results highlight the potential participation of the short utrophin isoform and the cleaved form of beta-dystroglycan in mdx Schwann cell membrane architecture. We proposed that these two proteins could be implicated in Schwann cell proliferation in response to a microenvironment stress such as mediated by accumulating macrophages in mdx mouse muscle inflammation sites.

Dystrophin splice variants are distinctly localized in the hippocampus

It has previously been demonstrated that Dp71, the most abundant dystrophin protein in the brain, is mainly localized in the postsynaptic densities. Here we show the localization of Dp71f, one of the splice variants of this protein, within the CA3 region of the hippocampus. Immunopositivity occurs in the postsynaptic density of small asymmetrical axospinous and axodendritic synapses, while it is absent in the postsynaptic densities of the axospinous synapses of the large mossy fiber terminals. Dp71f immunoreactivity was found to be attached to the membranes of the mossy fibers in the stratum lucidum of the CA3 area. In a certain population of thin myelinated axons the protein seems to be present within the axon proper. These data support the notion of a physiological role of Dp71f distinct from other dystrophin isoforms present in the central nervous system.

Pathological pattern of Mdx mice diaphragm correlates with gradual expression of the short utrophin isoform Up71

Utrophin gene is transcribed in a large mRNA of 13 kb that codes for a protein of 395 kDa. It shows amino acid identity with dystrophin of up to 73% and is widely expressed in muscle and non-muscle tissues. Up71 is a short utrophin product of the utrophin gene with the same cysteine-rich and C-terminal domains as full-length utrophin (Up395). Using RT-PCR, Western blots analysis, we demonstrated that Up71 is overexpressed in the mdx diaphragm, the most pathological muscle in dystrophin-deficient mdx mice, compared to wild-type C57BL/10 or other mdx skeletal muscles. Subsequently, we demonstrated that this isoform displayed an increased expression level up to 12 months, whereas full-length utrophin (Up395) decreased. In addition, beta-dystroglycan, the transmembrane glycoprotein that anchors the cytoplasmic C-terminal domain of utrophin, showed similar increase expression in mdx diaphragm, as opposed to other components of the dystrophin-associated protein complex (DAPC) such as alpha-dystrobrevin1 and alpha-sarcoglycan. We demonstrated that Up71 and beta-dystroglycan were progressively accumulated along the extrasynaptic region of regenerating clusters in mdx diaphragm. Our data provide novel functional insights into the pathological role of the Up71 isoform in dystrophinopathies.

A novel isoform of delta-sarcoglycan is localized at the sarcoplasmic reticulum of mouse skeletal muscle

The sarcoglycan-sarcospan complex (alpha-, beta-, gamma-, delta-, epsilon-, and zeta-SG-SSPN), a component of the dystrophin-associated glycoprotein complex (DAGC), is located at the sarcolemma of muscle fibers where it contributes to maintain cell integrity during contraction-relaxation cycles; gamma- and delta-SG are also expressed in the sarcoplasmic reticulum (SR). In this study, we report the identification of a novel isoform of murine delta-SG produced by alternative splicing that we named delta-SG3. This isoform is present at transcript level in several tissues, with its highest expression in skeletal and cardiac muscle. The delta-SG3 protein lacks the last 122 amino acids at the C-terminal, which are replaced by 10 new amino acids (EGFLNMQLAG). Interestingly, double immunofluorescence analysis for delta-SG3 and the dihydropyridine receptor (DHPR) shows a close localization of these two proteins. We propose the subcellular distribution of this novel delta-SG3 isoform at the SR and its involvement in intracellular calcium concentration regulation.

The sarcoglycan-sarcospan complex localization in mouse retina is independent from dystrophins

The sarcoglycan-sarcospan (SG-SSPN) complex is part of the dystrophin-glycoprotein complex that has been extensively characterized in muscle. To establish the framework for functional studies of sarcoglycans in retina here, we quantified sarcoglycans mRNA levels with real-time reverse transcriptase-polymerase chain reaction (RT-PCR) and performed immunohistochemistry to determine their cellular and subcellular distribution. We showed that the beta-, delta-, gamma-, epsilon-sarcoglycans and sarcospan are expressed in mouse retina. They are localized predominantly in the outer and the inner limiting membranes, probably in the Müller cells and also in the ganglion cells axons where the expression of dystrophins have never been reported. We also investigated the status of the sarcoglycans in the retina of mdx(3cv) mutant mice for all Duchene Muscular Dystrophy (DMD) gene products. The absence of dystrophin did not produce any change in the sarcoglycan-sarcospan components expression and distribution.

Dystrophin Dp71 in PC12 cell adhesion

Previously, we reported that PC12 cells with decreased Dp71 expression (antisense-Dp71 cells) display deficient nerve-growth-factor-induced neurite outgrowth. In this study, we show that disturbed neurite outgrowth of antisense-Dp71 cells is accompanied by decreased adhesion activity on laminin, collagen and fibronectin. In wild-type cells, the immunostaining of Dp71 and beta1-integrin overlaps in the basal area contacting the substrate, but staining of both proteins decrease in the antisense-Dp71 cells. Morphology of antisense-Dp71 cells at the electron microscopic level is characterized by the lack of filopodia, cellular projections involved in adhesion. Our findings suggest that Dp71 is required for the efficient PC12 cell attachment to beta1-integrin-dependent substrata and that decreased adhesion activity of the antisense-Dp71 cells could determine their deficiency to extend neurites.

The effect of respiratory muscle training with CO2 breathing on cellular adaptation of mdx mouse diaphragm

The aim of our study was to investigate the cellular mechanisms induced by hypercapnic stimulation of ventilation, during 6 weeks/30 min per day, in 10 mdx and 8 C57BL10 mice (10+/-0.2 months old). Ten mdx and eight C57BL10 mice served as control group. This respiratory training increases in vitro maximal tetanic tension of the diaphragm only in mdx mice. Western blot analysis of diaphragm showed: (1) an over-expression of alpha-dystrobrevin in mdx and C57BL10 training group compared to control group (8100+/-710 versus 6100+/-520 and 2800+/-400 versus 2200+/-250 arbitrary units); (2) a decrease in utrophin expression only in mdx training group compared to control group (2100+/-320 versus 3100+/-125 arbitrary units). Daily respiratory muscle training in mdx mice, induces a beneficial effect on diaphragm strength, with an over-expression of alpha-dystrobrevin. Further studies are needed to determine if, in absence of dystrophin, the over-expression of alpha-dystrobrevin could be interpreted as a possible pathway to improve function of dystrophic muscle.

alpha7B integrin changes in mdx mouse muscles after L-arginine administration

Muscle fibers attach to laminin in the basal lamina using two mechanisms, i.e., dystrophin with its associated proteins and alpha7beta1 integrin. In humans, gene-mutation defects in one member of these complexes result in muscular dystrophies. This study revealed changes after L-arginine treatment of utrophin-associated proteins and the alpha7B integrin subunit in mdx mouse, a dystrophin-deficient animal model. In the two studied muscles (cardiac muscle and diaphragm), the alpha7B integrin subunit was increased in 5-week-old treated mice. Interestingly, the diaphragm histopathological appearance was significantly improved by L-arginine administration. These results highlight a possible way to compensate for dystrophin deficiency via alpha7beta1 integrin.

Absence of Dp71 in mdx3cv mouse spermatozoa alters flagellar morphology and the distribution of ion channels and nNOS

In muscle, the absence of dystrophin alters the dystrophin-associated protein complex (DAPC), which is involved in the clustering and anchoring of signaling proteins and ion and water channels. Here we show that mice spermatozoa express only dystrophin Dp71 and utrophin Up71. The purpose of this study was to explore the effect of the absence of Dp71 on the morphology and membrane distribution of members of the DAPC, ion channels and signaling proteins of spermatozoa obtained from dystrophic mutant mdx3cv mice. Our work indicates that although the absence of Dp71 results in a dramatic decrease in beta-dystroglycan, it induces membrane redistribution and an increase in the total level of alpha-syntrophin, voltage-dependent Na+ (micro1) and K+ (Kv1.1) channels and neural nitric oxide synthase (nNOS). The short utrophin (Up71) was upregulated and redistributed in the spermatozoa of mdx3cv mice. A significant increase in abnormal flagella morphology was observed in the absence of Dp71, which was partially corrected when the plasma membrane was eliminated by detergent treatment. Our observations point to a new phenotype associated with the absence of Dp71. Abnormal flagellar structure and altered distribution of ion channels and signaling proteins may be responsible for the fertility problems of mdx3cv mice.

Monocarboxylate transporters, blood lactate removal after supramaximal exercise, and fatigue indexes in humans

The present study investigated whether muscular monocarboxylate transporter (MCT) 1 and 4 contents are related to the blood lactate removal after supramaximal exercise, fatigue indexes measured during different supramaximal exercises, and muscle oxidative parameters in 15 humans with different training status. Lactate recovery curves were obtained after a 1-min all-out exercise. A biexponential time function was then used to determine the velocity constant of the slow phase (gamma(2)), which denoted the blood lactate removal ability. Fatigue indexes were calculated during 1-min all-out (FI(AO)) and repeated 10-s (FI(Sprint)) cycling sprints. Biopsies were taken from the vastus lateralis muscle. MCT1 and MCT4 contents were quantified by Western blots, and maximal muscle oxidative capacity (V(max)) was evaluated with pyruvate + malate and glutamate + malate as substrates. The results showed that the blood lactate removal ability (i.e., gamma(2)) after a 1-min all-out test was significantly related to MCT1 content (r = 0.70, P < 0.01) but not to MCT4 (r = 0.50, P > 0.05). However, greater MCT1 and MCT4 contents were negatively related with a reduction of blood lactate concentration at the end of 1-min all-out exercise (r = -0.56, and r = -0.61, P < 0.05, respectively). Among skeletal muscle oxidative indexes, we only found a relationship between MCT1 and glutamate + malate V(max) (r = 0.63, P < 0.05). Furthermore, MCT1 content, but not MCT4, was inversely related to FI(AO) (r = -0.54, P < 0.05) and FI(Sprint) (r = -0.58, P < 0.05). We concluded that skeletal muscle MCT1 expression was associated with the velocity constant of net blood lactate removal after a 1-min all-out test and with the fatigue indexes. It is proposed that MCT1 expression may be important for blood lactate removal after supramaximal exercise based on the existence of lactate shuttles and, in turn, in favor of a better tolerance to muscle fatigue.

Immunofluorescence mapping of dystrophin in the rat brain: astrocytes contain the splice variant Dp71f, but this is confined to subpopulations

Dystrophins are membrane-associated actin-binding proteins, recognized at first in muscular dystrophies. In the brain the full-length Dp427 has been detected, as well as Dp140 and Dp71 of the shorter variants. Dp71 seems to be their major representative in the brain, and it occurs as splice variants, Dp71f and Dp71d. Dystrophins have been demonstrated mainly in neurons. In tissue cultures, the glial data, mainly in situ, are still insufficient. The present mapping study reveals the astroglial localization of the splice variant Dp71f, using a monoclonal antibody (5F3, developed by D. Mornet) specific for its additional 31 last amino acids. In parallel, another monoclonal antibody was used (Dys2, Novocastra) that detects the Dp71d, Dp427, as well as Dp140 and other short variants. Rats were overdosed with ether and perfused transcardially with 4% phosphate-buffered paraformaldehyde solution. Floating Vibratome sections were processed for immunohistochemical labeling with fluorescent secondary antibodies. In some animals the reactive glia were investigated following stab wounds in ketamine-xylazine anesthesia. Only the 5F3 antibody labeled astrocytes, however, not in general but in special localizations, mainly along the glia limitans of the pial surface, below the ependyma and in the reactive glia. Perivascular astrocytes were consistently labeled only where the vessels entered the brain, and in some circumventricular organs. The 5F3 antibody also labeled the ependyma and the residual subventricular zone. In contrast to the astrocytes, neurons were labeled throughout the brain. Dys2 antibody (to Dp71d and longer isoforms) labeled neurons in a distribution similar to that of 5F3, but rarely labeled astroglia and only in perivascular rings. Dp71f positivity seems to occur in those astrocyte populations that proved to be immunopositive to glial fibrillary acidic protein (GFAP) and produced laminin in former studies.

Formation of multiple complexes between beta-dystroglycan and dystrophin family products

Beta-dystroglycan is expressed in a wide variety of tissues and has generally been reported with an Mr of 43 kDa, sometimes accompanied with a 31 kDa protein assumed to be a truncated product. This molecule was recently identified as the anomalous beta-dystroglycan expressed in various carcinoma cell lines. We produced and characterized a G5 polyclonal antibody specific to beta-dystroglycan that is directed against the C-terminal portion of the molecule. We provide evidence that beta-dystroglycan may vary in size and properties by studying different Xenopus tissues. Besides normal beta-dystroglycan with an Mr of 43 kDa in smooth and cardiac muscle and sciatic nerve extracts, we found it in skeletal muscle and brain proteins with an Mr of 38 and 65 kDa, respectively. Glycosylation properties and proteolytic susceptibilities of these different beta-dystroglycans are analysed and compared in this work. Crosslinking experiments with various beta-dystroglycan preparations obtained from skeletal and cardiac muscles and brain gave rise to specific new covalent products with Mr of 125 kDa (doublet band), or 120 and 130 kDa, or 140 and 240 kDa, respectively. We provide evidence, using various similar beta-dystroglycan preparations, that the immunoprecipitation procedure with G5 specific polyclonal antibody allows consistent pelleting of various dystrophin-family isoforms. Skeletal muscles from Xenopus reveals the presence of two distinct beta-dystroglycan complexes, one with dystrophin and another one which involves alpha-dystrobrevin. Cardiac muscle and brain from Xenopus are shown to contain three beta-dystroglycan complexes related to various dystrophin-family isoforms. Dystrophin or alpha-dystrobrevin or Dp71 were found in cardiac muscle and dystrophin or Dp180 or Up71 in brain. This variability in the relationship between beta-dystroglycan and dystrophin-family isoforms suggests that each protein--currently known as dystrophin associated protein--could not be present in each of these complexes.

Dystrophin Dp71 is required for neurite outgrowth in PC12 cells

To determine the role of Dp71 in neuronal cells, we generated PC12 cell lines in which Dp71 protein levels were controlled by stable transfection with either antisense or sense constructs. Cells expressing the antisense Dp71 RNA (antisense-Dp71 cells) contained reduced amounts of the two endogenous Dp71 isoforms. Antisense-Dp71 cells exhibited a marked suppression of neurite outgrowth upon the induction with NGF or dibutyryl cyclic AMP. Early responses to NGF-induced neuronal differentiation, such as the cessation of cell division and the activation of ERK1/2 proteins, were normal in the antisense-Dp71 cells. On contrary, the induction of MAP2, a late differentiation marker, was disturbed in these cells. Additionally, the deficiency of Dp71 correlated with an altered expression of the dystrophin-associated protein complex (DAPC) members alpha and beta dystrobrevins. Our results indicate that normal expression of Dp71 is essential for neurite outgrowth in PC12 cells and constitute the first direct evidence implicating Dp71 in a neuronal function.

Subcellular localization of components of the dystrophin glycoprotein complex in cultured retinal muller glial cells

The dystrophin glycoprotein complex (DGC) is a membrane-associated protein complex binding extracellular matrix (ECM) molecules, such as laminin and forming a bridge towards the cytoskeleton. The molecular composition of the DGC is cell type dependent and it is involved in cell adhesion and motility. Here we present immunocytochemical localization of beta-dystroglycan, the central member of the DGC, utrophin and Dp71f, the spliced 71 kDa dystrophin protein product of the DMD gene, in cultured retinal Muller glial cells. It is shown that beta-dystroglycan and utrophin are colocalized in clusters in all parts of Muller cells including the lamellipodium and leading edge of migrating cells. As a contrast, Dp71f labels are distinct from beta-dystroglycan and confined to the perinuclear cytoplasm of Muller cells indicating that Dp71f is not a member of the DGC in cultured Muller cells.

Dystrophin and dystrophin-associated protein in muscles and nerves from monkey

Since all organs (i.e. skeletal, cardiac, smooth muscles and sciatic nerve) are never only taken from a single patient, all these tissues were obtained from one cynomolgus monkey, a model closely resembling humans. This work describes an up-to-date reinvestigation of the dystrophin-glycoprotein complex and related molecules in various monkey tissues such those cited above. We used monoclonal and polyclonal antibodies produced in our laboratory, which are directed against dystrophin, utrophin, short-dystrophin products, alpha-dystrobrevin, beta-dystroglycan, alpha-syntrophin, alpha-, beta-, gamma-, delta-, epsilon-sarcoglycan, and sarcospan. For each molecule, we determined their molecular weight and tissue localization. Regardless of the tissue analyzed, at least one dystrophin or utrophin as full-length molecule and one short-dystrophin product or dystrobrevin as proteins belonging to the dystrophin superfamily were found. Beta-dystroglycan, beta and delta sarcoglycans were always detected, while other sarcoglycans varied from all to only three components. Epsilon sarcoglycan appears to be specific to smooth muscle, which is devoid of alpha sarcoglycan. Sarcospan is only absent from sciatic nerve structures. Among the different muscles investigated in this study, short dystrophin products are only present in cardiac muscle. All of these findings are summarized in one table, which highlight in one single animal the variability of the dystrophin-glycoprotein complex components in relation with the organ studied. This statement is important because any attempt to estimate protein restoration needs in each study the knowledge of the expected components that should be considered normal.

Differential expression and subcellular distribution of dystrophin Dp71 isoforms during differentiation process

Dp71 is the major product of the Duchenne muscular dystrophy gene in the brain. In order to study the function of Dp71 in the nervous system we examined the expression of Dp71 isoforms in PC12 rat pheochromocytoma cell line, a well-established system to study neuronal differentiation. We show by reverse transcriptase-polymerase chain reaction and Western blot assays that PC12 cells express two Dp71 isoforms. One isoform lacks exon 71 and the other isoform lacks exons 71 and 78 (Dp71d and Dp71f isoforms respectively). Nerve growth factor-induced neuronal differentiation of PC12 cells results in differential regulation of the expression and subcellular localization of Dp71 isoforms: a) the amount of Dp71f protein increases nine-fold in total extracts while Dp71d increases up to seven-fold in nuclear extracts; b) Dp71f relocates from the cytoplasm to neuritic processes, being prominent at varicosities and the growth cone; c) Dp71d relocates almost entirely to the nucleus and is detected to a lower extent in the cytoplasm and neuritic processes. Dp71f co-localizes with beta-dystroglycan and synaptophysin while Dp71d co-localizes with beta-dystroglycan in the nucleus. Dp71d accumulates at cell-cell contacts where Dp71f is absent. These results suggest that Dp71d and Dp71f associate with different subcellular complexes and therefore may have distinct functions in PC12 cells.

Targeted inactivation of dystrophin gene product Dp71: phenotypic impact in mouse retina

The abnormal retinal neurotransmission observed in Duchenne muscular dystrophy (DMD) patients and in some genotypes of mice lacking dystrophin has been attributed to altered expression of short products of the dystrophin gene. We have investigated the potential role of Dp71, the most abundant C-terminal dystrophin gene product, in retinal electrophysiology. Comparison of the scotopic electroretinograms (ERG) between Dp71-null mice and wild-type (wt) littermates revealed a normal ERG in Dp71-null mice with no significant changes of the b-wave amplitude and kinetics. Analysis of DMD gene products, utrophin and dystrophin-associated proteins (DAPs), showed that Dp71 and utrophin were localized around the blood vessels, in the ganglion cell layer (GCL), and the inner limiting membrane (ILM). Dp71 deficiency was accompanied by an increased level of utrophin and decreased level of beta-dystroglycan localized in the ILM, without any apparent effect on the other DAPs. Dp71 deficiency was also associated with an impaired clustering of two Müller glial cell proteins-the inwardly rectifying potassium channel Kir4.1 and the water pore aquaporin 4 (AQP4). Immunostaining of both proteins decreased around blood vessels and in the ILM of Dp71-null mice, suggesting that Dp71 plays a role in the clustering and/or stabilization of the two proteins. AQP4 and Kir4.1 may also be involved in the regulation of the ischemic process. We found that a transient ischemia resulted in a greater damage in the GCL of mice lacking Dp71 than in wt mice. This finding points at a crucial role played by Dp71 in retinal function.

Dystrophin isoform Dp7l is present in lamellipodia and focal complexes in human astrocytoma cells U-373 MG

Dp71 is the most abundant product of the dmd gene in the brain. There are at least 2 isoforms derived from alternative splicing of exon 78 (Dp71d, which contains exon 78 and Dp71f, the spliced isoform) but the precise localization and function of each isoform is still unknown. In the present study, we demonstrate by RT-PCR that the Dp71f isoform is present in an astrocytoma cell line U-373 MG, and its subcellular localization was determined in the cytoplasm, particularly in perinuclear areas, with lower amounts towards the periphery but increasing in the leader borders of lamellipodia and focal complexes. Double labeling indirect immunofluorescence showed that Dp71f colocalized with actin-like beta-dystroglycan and beta-1 integrin. We also demonstrated by triple labeling that Dp71f was colocalized with actin and two members of integrin complexes, alpha-actinin and vinculin, in focal complexes. Ventral plasma membranes were enriched and in those containing focal complex proteins, we found colocalization of Dp71f, actin and vinculin. It is concluded that U-373 MG cells express Dp71f as part of lamellipodia and focal complex proteins, and possibly connected via distroglycan complexes to integrin complexes.

Comparative evolution of muscular dystrophy in diaphragm, gastrocnemius and masseter muscles from old male mdx mice

X chromosome-linked muscular dystrophic mdx mouse lacks the sarcolemmal protein dystrophin and represents a genetic homologue of human Duchenne muscular dystrophy (DMD). The present study analysed some aspects of pathological processes such as fibrosis, frequency of centralized nuclei, presence of degenerative or regenerative fibres, expression of utrophin and associated protein complexes, and myosin heavy chain isoforms in three muscles [diaphragm (DIA), gastrocnemius (GTC) and masseter (MAS)] from old male mdx mice. All parameters investigated comparatively in these pathological muscles provided evidence that the MAS mdx muscle presents a slight deterioration pattern in comparison to that of DIA and GTC muscles. Utrophin and associated proteins are present in many cell clusters with continuous membrane labelling in MAS muscle. Respective proportions of myosin heavy chain isoforms, measured by electrophoresis/densitometry, showed only slight change in GTC muscle, significant evolution in DIA muscle but drastic isoform conversions in MAS muscle. These results highlighted the difference in deterioration susceptibility of various muscles to muscular dystrophy. The reason why this occurs in MAS muscles is still obscure and discussed in terms of the comparative developmental origins of these muscles.

Comparative distribution of short dystrophin superfamily products in various guinea pig spermatozoa domains

In this study, the presence and cellular distribution of dystrophin family products (i.e. Dp71d, Dp71f-like protein and dystrobrevin) was examined by indirect immunofluorescence and Western blotting in guinea pig spermatozoa. Two dystrophin-associated proteins, beta-dystroglycan and alpha-syntrophin, and nNOS a protein frequently associated with alpha-syntrophin, were determined. In spermatozoa lacking plasma membrane and acrosome, Dp71f-like protein was found in the postacrosomal perinuclear theca and also in the middle piece of the flagellum. In the flagellum, Dp71f-like protein is localized together with alpha-syntrophin and nNOS. Dp71d was present in the plasma membrane of the middle piece with beta-dystroglycan, alpha-syntrophin and nNOS. Dp71d was also present in plasma membrane of the post acrosomal region, but only with nNOS. Finally, dystrobrevin was located all along skeletal flagellum structures and in the subacrosomal hemisphere of the perinuclear theca. This distinct and complementary distribution in various domains of spermatozoa may reveal a specific function for each short dystrophin family product, in the stabilization of the domains where they are located.

Neurocalcin-actin interaction

Neurocalcin is an N-myristoylated calcium-binding protein which belongs to a novel family of neuronal calcium sensors. Here we show, by cosedimentation, co-immunoprecipitation and cross-linking approaches, that myristoylated neurocalcin directly interacts with actin in a calcium-dependent manner. We used EDC cross-linking and obtained one novel 64 kDa entity composed of one actin molecule and one neurocalcin molecule, as demonstrated with IAEDANS-actin and neurocalcin-specific antibodies. This interaction could modulate the rod outer segment-guanylate cyclase 1-neurocalcin interface.

Calcium-dependent regulation of interactions of caldesmon with calcium-binding proteins found in growth cones of chick forebrain neurons

1. This study was undertaken to determine if caldesmon, calmodulin, S100beta, and neurocalcin delta were present in chick forebrain neurons, and if so, to investigate the interactions of these proteins in the presence of different concentrations of calcium. 2. Immunocytochemistry was used to determine the presence and localization of these proteins in cultured forebrain neurons. Western blotting, gel electrophoresis in the presence of different concentrations of calcium, chemical cross-linking, and affinity chromatography were used to investigate the interactions of these proteins with each other. 3. Our data show that caldesmon and three calcium-binding proteins (S100beta, calmodulin, and neurocalcin 3) are localized in growth cones and neurites of chick forebrain neurons in culture. In the presence of different concentration of calcium, these calcium-binding proteins have different affinities to caldesmon and to each other. S100beta binds with greater affinity than calmodulin to caldesmon, and its ability to bind to caldesmon is regulated by neurocalcin delta. 4. These findings suggest a specific calcium-dependent regulatory pathway for modulating actomyosin during growth cone motility.