Kyriaki Markoullis

Cyprus Institute of Neurology and Genetics, Lefkoşa, Lefkosia, Cyprus

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Publications (8)38.55 Total impact

  • No preview · Conference Paper · Aug 2015
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    ABSTRACT: Gap junctions are essential for glial cell function and have been increasingly implicated in multiple sclerosis (MS). Because increasing cortical abnormalities correlate with disease progression and cognitive dysfunction, we examined the expression of oligodendrocytic connexin32 (Cx32) and Cx47 and their astrocytic partners Cx30 and Cx43 in cortical lesions and normal-appearing gray matter (NAGM) in MS patients. Postmortem brain tissue samples from 9 MS cases were compared with 10 controls using real-time polymerase chain reaction, immunoblot, and immunohistochemical analyses. Connexin32 and Cx47 gap junction formation in oligodendrocytes was reduced within lesions, whereas Cx32 loss also extended to NAGM. In contrast, astrocytic Cx30 expression was increased within cortical lesions, whereas Cx43 was elevated in both lesions and NAGM. Diffuse microglial activation and marked astrogliotic changes accompanied these connexin abnormalities. Increased expression of Cx43 correlated with inflammatory load (r = 0.828, p = 0.042), whereas Cx32 expression correlated with longer disease duration and, therefore, milder course (r = 0.825, p = 0.043). Thus, there is a loss of intramyelin and intercellular oligodendrocyte gap junctions in MS gray matter lesions and NAGM, whereas interastrocytic gap junctions are increased, reflecting astrogliosis. These changes correlate with inflammation and disease duration and suggest that disconnection of oligodendrocytes from reactive astrocytes may play a role in failed remyelination and disease progression.
    Full-text · Article · Aug 2014 · Journal of Neuropathology and Experimental Neurology
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    Kleopas A. Kleopa · Irene Sargiannidou · Kyriaki Markoullis
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    ABSTRACT: Central nervous system (CNS) glia are extensively coupled through gap junctions (GJ) formed by a highly cell-specific set of connexins. Recently characterized inherited human disorders and respective animal models highlight the crucial role of glial GJ in CNS myelination and homeostasis. These emerging roles of connexins have generated interest into their possible involvement in common acquired demyelinating disorders, especially multiple sclerosis (MS). Analysis of post-mortem brain samples from MS patients, including morphological, biochemical, and gene expression studies, showed significant alteration in glial connexins not only in lesions, but also extending into the normal appearing white matter (NAWM). Compared with non-MS cases, the oligodendrocyte connexins, Cx32 and Cx47, were reduced in and around chronic MS lesions, whereas Cx43, the major astrocytic partner in oligodendrocyte–astrocyte (O/A) GJ, was increased, reflecting astrogliosis. Furthermore, in the NAWM, Cx32 GJ were significantly reduced along myelinated fibers, whereas Cx47 showed increased expression mainly in oligodendrocyte precursor cells (OPC). However, OPC showed only limited connectivity to astrocytes. These alterations were replicated in the experimental autoimmune encephalomyelitis (EAE) model, with persistent loss of Cx47 and Cx32 GJ even away from lesions. Additionally, Cx43 was severely reduced in acute EAE with disruption of O/A GJ, followed by increased Cx43 expression at chronic stages of astrogliosis, but without complete re-establishment of GJ to oligodendrocytes. Thus, loss of oligodendrocyte GJ in myelinated fibers, and disrupted O/A GJ connectivity in MS lesions and in NAWM in the setting of persistent inflammation and astrogliosis emerge as important aspects of MS pathology, and might have implications for disease progression.
    Preview · Article · Dec 2013 · Clinical and Experimental Neuroimmunology
  • Lida Zoupi · Kyriaki Markoullis · Kleopas A Kleopa · Domna Karagogeos
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    ABSTRACT: The segregation of myelinated fibers into distinct domains around the node of Ranvier-the perinodal areas-is crucial for nervous system homeostasis and efficient nerve conduction. Perinodal areas are formed by axo-glial interactions, namely the interaction of molecules between the axon and the myelinating glia. In a variety of demyelinating pathologies including multiple sclerosis, the molecular architecture of the myelinated fiber is disrupted, leading to axonal degeneration. In this study we have analyzed the alterations of TAG-1, Caspr2, and voltage-gated potassium channels (VGKCs), forming the juxtaparanodal tripartite complex, in relation to adjacent paranodal and nodal molecules, in two different models of CNS demyelination, the experimental autoimmune encephalomyelitis (EAE) and the cuprizone model of toxic demyelination. We found extensive alterations of the juxtaparanodal molecular architecture under de- and remyelinating conditions. Inflammation alone was sufficient to disrupt the borders between the domains leading to the diffusion of juxtaparanodal components to the adjacent paranodal area. EAE induction and cuprizone-induced demyelination resulted initially in paranodal domain elongation with subsequent diffusion of the juxtaparanodal components and the reduction of their expression levels. At later stages, with decreasing inflammation and spontaneous remyelination there was a partial restoration of the paranodal domain but not sufficient re-organization of the juxtaparanodes. The latter were re-formed only when complete remyelination was allowed in the cuprizone model, indicating that juxtaparanodal domain reorganization is a later event that may remain incomplete in a hostile inflammatory milieu.
    No preview · Article · Aug 2013 · Glia
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    ABSTRACT: Gap junctions (GJs) are vital for oligodendrocyte survival and myelination. In order to examine how different stages of inflammatory demyelination affect oligodendrocyte GJs, we studied the expression of oligodendrocytic connexin32 (Cx32) and Cx47 and astrocytic Cx43 in the experimental autoimmune encephalomyelitis (EAE) mouse model of multiple sclerosis (MS) induced by recombinant myelin oligodendrocyte glycoprotein. EAE was characterized by remissions and relapses with demyelination and axonal loss. Formation of GJ plaques was quantified in relation to the lesions and in normal appearing white matter (NAWM). During acute EAE at 14 days postimmunization (dpi) both Cx47 and Cx32 GJs were severely reduced within and around lesions but also in the NAWM. Cx47 was localized intracellularly in oligodendrocytes while protein levels remained unchanged, and this redistribution coincided with the loss of Cx43 GJs in astrocytes. Cx47 and Cx32 expression increased during remyelination at 28 dpi but decreased again at 50 dpi in the relapsing phase. Oligodendrocyte GJs remained reduced even in NAWM, despite increased formation of Cx43 GJs toward lesions indicating astrogliosis. EAE induced in Cx32 knockout mice resulted in an exacerbated clinical course with more demyelination and axonal loss compared with wild-type EAE mice of the same backcross, despite similar degree of inflammation, and an overall milder loss of Cx47 and Cx43 GJs. Thus, EAE causes persistent impairment of both intra- and intercellular oligodendrocyte GJs even in the NAWM, which may be an important mechanism of MS progression. Furthermore, GJ deficient myelinated fibers appear more vulnerable to CNS inflammatory demyelination.
    No preview · Article · Jul 2012 · Glia
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    ABSTRACT: Oligodendrocyte gap junctions (GJs) are vital for central nervous system myelination, but their involvement in multiple sclerosis (MS) pathology remains unknown. The aim of this study was to examine alterations of oligodendrocyte and related astrocyte GJs in MS lesions and normal-appearing white matter (NAWM). Post-mortem brain samples from 9 MS and 11 age-matched non-MS control patients were studied. Tissue sections that included both chronic active and inactive lesions were characterized neuropathologically with Luxol Fast Blue staining and immunostaining for myelin oligodendrocyte glycoprotein (MOG) and the microglial marker Iba1. We analyzed the expression of Cx32 and Cx47 in oligodendrocytes and of Cx43, the major astrocytic partner in oligodendrocyte-astrocyte (O/A) GJs by quantitative immunoblot and real-time PCR. Formation of GJ plaques was quantified by immunohistochemistry. Compared to control brains, both Cx32 and Cx47 GJ plaques and protein levels were reduced in and around MS lesions, while Cx43 was increased as part of astrogliosis. In the NAWM, Cx32 was significantly reduced along myelinated fibers whereas Cx47 showed increased expression mainly in oligodendrocyte precursor cells (OPCs). However, OPCs showed only limited connectivity to astrocytes. Cx43 showed modestly increased levels in MS NAWM compared to controls, while GJ plaque counts were unchanged. Our findings indicate that oligodendrocyte GJs are affected not only in chronic MS lesions but also in NAWM, where disruption of Cx32 GJs in myelinated fibers may impair myelin structure and function. Moreover, limited O/A GJ connectivity of recruited OPCs in the setting of persistent inflammation and astrogliosis may prevent differentiation and remyelination.
    Full-text · Article · Apr 2012 · Acta Neuropathologica
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    ABSTRACT: The X-linked demyelinating/type I Charcot-Marie-Tooth neuropathy (CMT1X) is an inherited peripheral neuropathy caused by mutations in GJB1, the gene that encodes the gap junction protein connexin32. Connexin32 is expressed by myelinating Schwann cells and forms gap junctions in noncompact myelin areas, but axonal involvement is more prominent in X-linked compared with other forms of demyelinating Charcot-Marie-Tooth disease. To clarify the cellular and molecular mechanisms of axonal pathology in CMT1X, we studied Gjb1-null mice at early stages (i.e. 2-4 months old) of the neuropathy, when there is minimal or no demyelination. The diameters of large myelinated axons were progressively reduced in Gjb1-null mice compared with those in wild-type littermates. Furthermore, neurofilaments were relatively more dephosphorylated and more densely packed starting at 2 months of age. Increased expression of β-amyloid precursor protein, a marker of axonal damage, was also detected in Gjb1-null nerves. Finally, fast axonal transport, assayed by sciatic nerve ligation experiments, was slower in distal axons of Gjb1-null versus wild-type animals with reduced accumulation of synaptic vesicle-associated proteins. These findings demonstrate that axonal abnormalities including impaired cytoskeletal organization and defects in axonal transport precede demyelination in this mouse model of CMT1X.
    Full-text · Article · Sep 2010 · Journal of Neuropathology and Experimental Neurology
  • Irene Sargiannidou · Kyriaki Markoullis · Kleopas A Kleopa
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    ABSTRACT: There is an emerging group of neurological disorders that result from genetic mutations affecting gap junction proteins in myelinating cells. The X-linked form of Charcot Marie Tooth disease (CMT1X) is caused by numerous mutations in the GJB1 gene encoding the gap junction protein connexin32 (Cx32), which is expressed in both Schwann cells in the PNS and oligodendrocytes in the CNS. Patients with CMT1X present mainly with a progressive peripheral neuropathy, showing mixed axonal and demyelinating features. In many cases there is also clinical or subclinical involvement of the CNS with acute or chronic phenotypes of encephalopathy. Furthermore, mutations in the GJA12/GJC2 gene encoding the gap junction protein Cx47, which is expressed in oligodendrocytes, have been identified in families with progressive leukodystrophy, known as Pelizaeus-Merzbacher-like disease, as well as in patients with hereditary spastic paraplegia. Recent studies have provided insights into the pattern of gap junction protein expression and function in CNS and PNS myelinating cells. Furthermore, in vitro and in vivo disease models have clarified some of the molecular and cellular mechanisms underlying these disorders. Here we provide an overview of the clinical, genetic, and neurobiological aspects of gap junction disorders affecting the nervous system.
    No preview · Article · Sep 2010 · Histology and histopathology