Characterization of NF-L and betaIISigma1-spectrin interaction in live cells

The Roles of Cystatin B in the Brain and Pathophysiological Mechanisms of Progressive Myoclonic Epilepsy Type 1

Progressive myoclonic epilepsy type 1 (EPM1) is an autosomal recessive disorder, also known as Unverricht-Lundborg disease (ULD). EPM1 patients suffer from photo-sensitive seizures, stimulus-sensitive myoclonus, nocturnal myoclonic seizures, ataxia and dysarthria. In addition, cerebral ataxia and impaired GABAergic inhibition are typically present. EPM1 is caused by mutations in the Cystatin B gene (CSTB). The CSTB protein functions as an intracellular thiol protease inhibitor and inhibits Cathepsin function. It also plays a crucial role in brain development and regulates various functions in neurons beyond maintaining cellular proteostasis. These include controlling cell proliferation and differentiation, synaptic functions and protection against oxidative stress, likely through regulation of mitochondrial function. Depending on the differentiation stage and status of neurons, the protein localizes either to the cytoplasm, nucleus, lysosomes or mitochondria. Further, CSTB can also be secreted to the extracellular matrix for interneuron rearrangement and migration. In this review, we will review the various functions of CSTB in the brain and discuss the putative pathophysiological mechanism underlying EPM1.

Cytoskeleton as a Target of Quinolinic Acid Neurotoxicity: Insight from Animal Models

Cytoskeletal proteins are increasingly recognized as having important roles as a target of the action of different neurotoxins. In the last years, several works of our group have shown that quinolinic acid (QUIN) was able to disrupt the homeostasis of the cytoskeleton of neural cells and this was associated with cell dysfunction and neurodegeneration. QUIN is an excitotoxic metabolite of tryptophan metabolism and its accumulation is associated with several neurodegenerative diseases. In the present review, we provide a comprehensive view of the actions of QUIN upstream of glutamate receptors, eliciting kinase/phosphatase signaling cascades that disrupt the homeostasis of the phosphorylation system associated with intermediate filament proteins of astrocytes and neurons. We emphasize the critical role of calcium in these actions and the evidence that misregulated cytoskeleton takes part of the cell response to the injury resulting in neurodegeneration in different brain regions, disrupted cell signaling in acute tissue slices, and disorganized cytoskeleton with altered cell morphology in primary cultures. We also discuss the interplay among misregulated cytoskeleton, oxidative stress, and cell-cell contact through gap junctions mediating the quinolinic acid injury in rat brain. The increasing amount of cross talks identified between cytoskeletal proteins and cellular signaling cascades reinforces the exciting possibility that cytoskeleton could be a new target in the neurotoxicity of QUIN and further studies will be necessary to develop strategies to protect the cytoskeleton and counteracts the cytotoxicity of this metabolite.

Neurofilament dynamics and involvement in neurological disorders

Neurons are extremely polarised cells in which the cytoskeleton, composed of microtubules, microfilaments and neurofilaments, plays a crucial role in maintaining structure and function. Neurofilaments, the 10-nm intermediate filaments of neurons, provide structure and mechanoresistance but also provide a scaffolding for the organization of the nucleus and organelles such as mitochondria and ER. Disruption of neurofilament organization and expression or metabolism of neurofilament proteins is characteristic of certain neurological syndromes including Amyotrophic Lateral Sclerosis, Charcot-Marie-Tooth sensorimotor neuropathies and Giant Axonal Neuropathy. Microfluorometric live imaging techniques have been instrumental in revealing the dynamics of neurofilament assembly and transport and their functions in organizing intracellular organelle networks. The insolubility of neurofilament proteins has limited identifying interactors by conventional biochemical techniques but yeast two-hybrid experiments have revealed new roles for oligomeric, nonfilamentous structures including vesicular trafficking. Although having long half-lives, new evidence points to degradation of subunits by the ubiquitin-proteasome system as a mechanism of normal turnover. Although certain E3-ligases ubiquitinating neurofilament proteins have been identified, the overall process of neurofilament degradation is not well understood. We review these mechanisms of neurofilament homeostasis and abnormalities in motor neuron and peripheral nerve disorders. Much remains to discover about the disruption of processes that leads to their pathological aggregation and accumulation and the relevance to pathogenesis. Understanding these mechanisms is crucial for identifying novel therapeutic strategies.

Myosin Va but Not nNOSα is Significantly Reduced in Jejunal Musculomotor Nerve Terminals in Diabetes Mellitus

Nitric oxide (NO) mediated slow inhibitory junction potential and mechanical relaxation after electrical field stimulation (EFS) is impaired in diabetes mellitus. Externally added NO donor restore nitrergic function, indicating that this reduction result from diminution of NO synthesis within the pre-junctional nerve terminals. The present study aimed to investigate two specific aims that may potentially provide pathophysiological insights into diabetic nitrergic neuropathy. Specifically, alteration in nNOSα contents within jejunal nerve terminals and a local subcortical transporter myosin Va was tested 16 weeks after induction of diabetes by low dose streptozotocin (STZ) in male Wistar rats. The results show that diabetic rats, in contrast to vehicle treated animals, have: (a) nearly absent myosin Va expression in nerve terminals of axons innervating smooth muscles and (b) significant decrease of myosin Va in neuronal soma of myenteric plexus. In contrast, nNOSα staining in diabetic jejunum neuromuscular strips showed near intact expression in neuronal cell bodies. The space occupancy of nitrergic nerve fibers was comparable between groups. Normal concentration of nNOSα was visualized within a majority of nitrergic terminals in diabetes, suggesting intact axonal transport of nNOSα to distant nerve terminals. These results reveal the dissociation between presences of nNOSα in the nerve terminals but deficiency of its transporter myosin Va in the jejunum of diabetic rats. This significant observation of reduced motor protein myosin Va within jejunal nerve terminals may potentially explain impairment of pre-junctional NO synthesis during EFS of diabetic gut neuromuscular strips despite presence of the nitrergic synthetic enzyme nNOSα.

Giant axon formation in mice lacking Kell, XK, or Kell and XK: animal models of McLeod neuroacanthocytosis syndrome

McLeod neuroacanthocytosis syndrome (MLS) is a rare X-linked multisystem disease caused by XK gene mutations and characterized by hematological and neurological abnormalities. XK, a putative membrane transporter, is expressed ubiquitously and is covalently linked to Kell, an endothelin-3-converting enzyme (ECE-3). Absence of XK results in reduction of Kell at sites where both proteins are coexpressed. To elucidate the functional roles of XK, Kell, and the XK-Kell complex associated with pathogenesis in MLS, we studied the pathology of the spinal cord, anterior roots, sciatic nerve, and skeletal muscle from knockout mouse models, using Kel(-/-), Xk(-/-), Kel(-/-)Xk(-/-), and wild-type mice aged 6 to 18 months. A striking finding was that giant axons were frequently associated with paranodal demyelination. The pathology suggests probable anterograde progression from the spinal cord to the sciatic nerve. The neuropathological abnormalities were found in all three genotypes, but were more marked in the double-knockout Kel(-/-)Xk(-/-) mice than in either Kel(-/-) or Xk(-/-) mice. Skeletal muscles from Xk(-/-) and Kel(-/-)Xk(-/-) mice showed mild abnormalities, but those from Kel(-/-) mice were similar to the wild type. The more marked neuropathological abnormalities in Kel(-/-)Xk(-/-) mice suggest a possible functional association between XK and Kell in nonerythroid tissues.

Protein 4.1B contributes to the organization of peripheral myelinated axons

Neurons are characterized by extremely long axons. This exceptional cell shape is likely to depend on multiple factors including interactions between the cytoskeleton and membrane proteins. In many cell types, members of the protein 4.1 family play an important role in tethering the cortical actin-spectrin cytoskeleton to the plasma membrane. Protein 4.1B is localized in myelinated axons, enriched in paranodal and juxtaparanodal regions, and also all along the internodes, but not at nodes of Ranvier where are localized the voltage-dependent sodium channels responsible for action potential propagation. To shed light on the role of protein 4.1B in the general organization of myelinated peripheral axons, we studied 4.1B knockout mice. These mice displayed a mildly impaired gait and motility. Whereas nodes were unaffected, the distribution of Caspr/paranodin, which anchors 4.1B to the membrane, was disorganized in paranodal regions and its levels were decreased. In juxtaparanodes, the enrichment of Caspr2, which also interacts with 4.1B, and of the associated TAG-1 and Kv1.1, was absent in mutant mice, whereas their levels were unaltered. Ultrastructural abnormalities were observed both at paranodes and juxtaparanodes. Axon calibers were slightly diminished in phrenic nerves and preterminal motor axons were dysmorphic in skeletal muscle. βII spectrin enrichment was decreased along the axolemma. Electrophysiological recordings at 3 post-natal weeks showed the occurrence of spontaneous and evoked repetitive activity indicating neuronal hyperexcitability, without change in conduction velocity. Thus, our results show that in myelinated axons 4.1B contributes to the stabilization of membrane proteins at paranodes, to the clustering of juxtaparanodal proteins, and to the regulation of the internodal axon caliber.

Cell cycle deregulation and loss of stem cell phenotype in the subventricular zone of TGF-beta adaptor elf-/- mouse brain

The mammalian forebrain subependyma contains neural stem cells and other proliferating progenitor cells. Recent studies have shown the importance of TGF-beta family members and their adaptor proteins in the inhibition of proliferation in the nervous system. Previously, we have demonstrated that TGF-beta induces phosphorylation and association of ELF (embryonic liver fodrin) with Smad3 and Smad4 resulting in nuclear translocation. Elf(-/-) mice manifest abnormal neuronal differentiation, with loss of neuroepithelial progenitor cell phenotype in the subventricular zone (SVZ) with dramatic marginal cell hyperplasia and loss of nestin expression. Here, we have analyzed the expression of cell cycle-associated proteins cdk4, mdm2, p21, and pRb family members in the brain of elf(-/-) mice to verify the role of elf in the regulation of neural precursor cells in the mammalian brain. Increased proliferation in SVZ cells of the mutant mice coincided with higher levels of cdk4 and mdm2 expression. A lesser degree of apoptosis was observed in the mutant mice compared to the wild-type control. Elf(-/-) embryos showed elevated levels of hyperphosphorylated forms of pRb, p130 and p107 and decreased level of p21 compared to the wild-type control. These results establish a critical role for elf in the development of a SVZ neuroepithelial stem cell phenotype and regulation of neuroepithelial cell proliferation, suggesting that a mutation in the elf locus renders the cells susceptible to a faster entry into S phase of cell cycle and resistance to senescence and apoptotic stimuli.

Specific interaction of the actin-binding domain of dystrophin with intermediate filaments containing keratin 19

Cytokeratins 8 and 19 concentrate at costameres of striated muscle and copurify with the dystrophin-glycoprotein complex, perhaps through the interaction of the cytokeratins with the actin-binding domain of dystrophin. We overexpressed dystrophin's actin-binding domain (Dys-ABD), K8 and K19, as well as closely related proteins, in COS-7 cells to assess the basis and specificity of their interaction. Dys-ABD alone associated with actin microfilaments. Expressed with K8 and K19, which form filaments, Dys-ABD associated preferentially with the cytokeratins. This interaction was specific, as the homologous ABD of betaI-spectrin failed to interact with K8/K19 filaments, and Dys-ABD did not associate with desmin or K8/K18 filaments. Studies in COS-7 cells and in vitro showed that Dys-ABD binds directly and specifically to K19. Expressed in muscle fibers in vivo, K19 accumulated in the myoplasm in structures that contained dystrophin and spectrin and disrupted the organization of the sarcolemma. K8 incorporated into sarcomeres, with no effect on the sarcolemma. Our results show that dystrophin interacts through its ABD with K19 specifically and are consistent with the idea that cytokeratins associate with dystrophin at the sarcolemma of striated muscle.

Cloning and characterization of cytokeratins 8 and 19 in adult rat striated muscle. Interaction with the dystrophin glycoprotein complex

We used degenerate primers for the amino- and carboxyl-terminal ends of the rod domains of intermediate filament proteins in reverse transcriptase-PCR experiments to identify and clone cytokeratins 8 and 19 (K8 and K19) from cardiac muscle of the adult rat. Northern blots showed that K8 has a 2.2-kb transcript and K19 has a 1.9-kb transcript in both adult cardiac and skeletal muscles. Immunolocalization of the cytokeratins in adult cardiac muscle with isoform-specific antibodies for K8 and K19 showed labeling at Z-lines within the muscle fibers and at Z-line and M-line domains at costameres at the sarcolemmal membrane. Dystrophin and K19 could be co-immunoprecipitated and co-purified from extracts of cardiac muscle, suggesting a link between the cytokeratins and the dystrophin-based cytoskeleton at the sarcolemma. Furthermore, transfection experiments indicate that K8 and K19 may associate with dystrophin through a specific interaction with its actin-binding domain. Consistent with this observation, the cytokeratins are disrupted at the sarcolemmal membrane of skeletal muscle of the mdx mouse that lacks dystrophin. Together these results indicate that at least two cytokeratins are expressed in adult striated muscle, where they may contribute to the organization of both the myoplasm and sarcolemma.

Axonal transport of membranous and nonmembranous cargoes: a unified perspective

Membranous and nonmembranous cargoes are transported along axons in the fast and slow components of axonal transport, respectively. Recent observations on the movement of cytoskeletal polymers in axons suggest that slow axonal transport is generated by fast motors and that the slow rate is due to rapid movements interrupted by prolonged pauses. This supports a unified perspective for fast and slow axonal transport based on rapid movements of diverse cargo structures that differ in the proportion of the time that they spend moving. A Flash feature (http://www.jcb.org/cgi/content/full/jcb.200212017/DC1) accompanies this Mini-Review.

Sarcolemmal organization in skeletal muscle lacking desmin: evidence for cytokeratins associated with the membrane skeleton at costameres

The sarcolemma of fast-twitch muscle is organized into "costameres," structures that are oriented transversely, over the Z and M lines of nearby myofibrils, and longitudinally, to form a rectilinear lattice. Here we examine the role of desmin, the major intermediate filament protein of muscle in organizing costameres. In control mouse muscle, desmin is enriched at the sarcolemmal domains that lie over nearby Z lines and that also contain beta-spectrin. In tibialis anterior muscle from mice lacking desmin due to homologous recombination, most costameres are lost. In myofibers from desmin -/- quadriceps, by contrast, most costameric structures are stable. Alternatively, Z line domains may be lost, whereas domains oriented longitudinally or lying over M lines are retained. Experiments with pan-specific antibodies to intermediate filament proteins and to cytokeratins suggest that control and desmin -/- muscles express similar levels of cytokeratins. Cytokeratins concentrate at the sarcolemma at all three domains of costameres when the latter are retained in desmin -/- muscle and redistribute with beta-spectrin at the sarcolemma when costameres are lost. Our results suggest that desmin associates with and selectively stabilizes the Z line domains of costameres, but that cytokeratins associate with all three domains of costameres, even in the absence of desmin.