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David Paul, PhD
Professor of Neurobiology
Harvard Medical School/Children's Hospital






MRRC Project(s)

R01 GM37751-14
Function and Regulation of Intercellular Communication

Gap Junctions are composed of intercellular channels that permit the direct exchange of ions and small molecules between adjacent cells. These channels are formed from connexins, a family of at least 19 genes. Mutations in human connexin (Cx) genes have been associated with neurodegenerative diseases, deafness, cataracts, skin disorders and cardiovascular abnormalities. Thus, gap junctions contribute to a wide range of cellular functions.

Research Description

Highlights of Major Accomplishments

Previously, we showed that mutations in the gene encoding connexin32 (Cx32) caused a demyelinating peripheral neuropathy called Charcot-Marie-Tooth disease (CMTX). Consistent with this finding, Schwann cells contain Cx32 and regulate its expression like a myelin-related gene. Thus, maintenance of myelin in the human peripheral nervous system requires connexin expression. However, oligodendrocytes also express and regulate Cx32 like a myelin gene and yet central abnormalities are rare in CMTX patients. Since one explanation for this discrepancy would be redundant expression of other connexins, we searched for connexins in myelinating glia. We found two novel connexins, Cx29 and Cx47. All three connexins are expressed in both oligodendrocytes and Schwann cells. Cx29 and Cx32, however, are present in non-overlapping subsets of spinal cord oligodendrocytes and while they are both present in Schwann cells, their subcellular distributions are strikingly different. On the other hand, Cx47 largely overlaps with Cx32, at least in oligodendrocytes. Single knockouts of either Cx29, Cx32 or Cx47 myelinate normally and have no functional deficits. In contrast, double knockouts in Cx32 and Cx47 display massive central demyelination accompanied by oligodendrocyte cell death and axonal loss leading to death by the 6th postnatal week. Surprisingly, these animals display only subtle abnormalities in peripheral myelin.

There appear to be significant differences in the way that oligodendrocytes and Schwann cells use connexins. For example, myelinating Schwann cells do not establish gap junctions with neighboring cells; we and others postulate that Schwann cells may form 'reflexive' junctions that couple different regions of the same cell. Oligodendrocytes also do not form gap junctions with other oligodendrocytes but do establish large numbers of junctions with neighboring astrocytes. Astrocytes, in turn, express a set of three additional connexins and establish gap junctions with each other, potentially forming an extensive network of coupled cells into which oligodendrocytes are recruited. Together, our studies suggest that connexins are critical for both central and peripheral myelination, that different connexins have different functions within myelinating glia, We are defining the separate and interacting roles of glial connexins using a combination of targeted gene ablation, cell biological and electrophysiological approaches.

Another interest of our lab is the use of gap junctions as electrical synapses by neurons. In the retina, for example, gap junctions/electrical synapses are established by most cell types and may participate in retinal circuitry a variety of ways. Using a Cx36 knockout (KO) mouse incorporating a histochemical reporter, we found Cx36 to be present in most or all AII amacrine cells, many photoreceptors, subsets of ON and OFF cone bipolar cells and a low number of ganglion cells. However, horizontal cells and many ganglion cells establish gap junctions yet do not express Cx36. Using RT-PCR, we have found Cx45 and Cx57 to be prominently expressed in mouse retina. We are currently characterizing the cellular distribution of these connexins.

The distribution of Cx36 suggesting it could be involved in rod photoreceptor signaling. In this regard, several lines of evidence indicate that rods signals utilize multiple pathways to reach ganglion cells. In our Cx36 KO, ON ganglion cell responses to scotopic stimuli are completely eliminated, indicating that Cx36 is required in all pathways contributing to rod ON signaling. We showed that gap junctional coupling between AII and cone ON bipolar cells, the presumed 'primary' pathway for rod photoreceptor signaling, was abolished in the KO. Since a proposed 'alternative' pathway involved gap junction between rods and cones, and since we found Cx36 expression in photoreceptors, our findings were consistent with the proposed 'alternative' pathway. However, recent studies of retinas from mice genetically altered to lack cones suggest that rod-cone coupling does not contribute to rod photoreceptor signaling. In addition, it has not been technically feasible to measure junctional coupling between photoreceptors in mouse retinas. Therefore, to resolve the discrepancy between current models of the 'alternative' pathway, we are developing methods to directly assess rod-cone coupling in WT and Cx36 KO mice and producing conditional knockouts that eliminate Cx36 specifically from either cones or from AII amacrine cells. Then, ganglion cell responses to light can be assessed in retinas where either primary or alternative pathways are ablated.

Publications

Rouan F, White TW, Brown N, Taylor AM, Lucke TW, Paul DL, Munro C, Uitto J, Hodgins MB, Richard G. Trans-dominant inhibition of connexin-43 by mutant connexin-26: implications for dominant connexin disorders affecting epidermal differentiation. J Cell Sci 2001;114:2105-2113.

Kasahara H, Wakimoto H, Converso KL, Maguire CT, Vargas HMM, Manning WJ, Paul DL, Lawitts J, Berul CI, Izumo S. Progressive atrioventricular conduction defects and heart failure in mice expressing Csx/Nkx2.5 homeodomain mutant protein. J ClinInvest 2001;108:189-201.

White TW, Sellitto C, Paul DL, Goodenough DA. Connexin43 and connexin50 are not required for prenatal lens development. Invest Ophthalmol Vis Sci 2001;42:2916-2923.
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Deans MR, Gibson JR, Sellitto C, Connors BW, Paul DL. Synchronous activity in neocortical inhibitory networks is dependent on electrical synapses containing Connexin36. Neuron 2001;31:477-485.

Deans MR, Paul DL. Mouse horizontal cells do not express connexin26 or connexin36. Cell Commun Adhes 2001;8(4-6):361-6.

Landisman CE, Long ME, Beierlein M, Deans MR, Paul DL, Connors BW. Electrical synapses in the thalamic reticular nucleus. J Neurosci 2002;22:1002-1009.

Deans MR, Volgyi B, Goodenough DA, Bloomfield SA, Paul DL. Connexin36 is essential for transmission of rod-mediated visual signals in the mammalian retina. Neuron 2002;36(4):703-12.

Altevogt BM, Kleopa KA, Postma FR, Scherer SS, Paul DL. Connexin29 is uniquely distributed within myelinating glial cells of the central and peripheral nervous systems. J Neurosci 2002;22:6458-6470.

Landesman Y, Goodenough DA, Paul DL. Xwnt-2 (Xwnt-2b) is maternally expressed in Xenopus oocytes and embryos. Biochim Biophys Acta 2002;1576:265-268.

Blinder KJ, Pumplin DW, Paul DL, Keller A. Intercellular interactions in the mammalian olfactory nerve. J Comp Neurol 2003;466:230-239.

Menichella DM, Goodenough DA, Sirkowski E, Scherer SS, Paul DL. Connexins are critical for normal myelination in the CNS. J Neurosci 2003;23:5963-5973.

Kasahara H, Ueyama T, Wakimoto H, Liu MK, Maguire CT, Converso KL, Kang PM, Manning WJ, Lawitts J, Paul DL, Berul CI, Izumo S. Nkx2.5 homeoprotein regulates expression of gap junction protein connexin 43 and sarcomere organization in postnatal cardiomyocytes. J Mol Cell Cardiol 2003;35:243-256.

Goodenough DA, Paul DL. Beyond the gap: functions of unpaired connexon channels. Nat Rev Mol Cell Biol 2003;4:285-294. 15. Figueroa XF, Paul DL, Simon AM, Goodenough DA, Day KH, Damon DN, Duling BR. Central role of connexin40 in the propagation of electrically activated vasodilation in mouse cremasteric arterioles in vivo. Circ Res 2003;92:793-800.

Melanson-Drapeau L, Beyko S, Dave S, Hebb AL, Franks DJ, Sellitto C, Paul DL, Bennett SA. Oligodendrocyte progenitor enrichment in the connexin32 null-mutant mouse. J Neurosci 2003;23:1759-1768.

Landesman Y, Postma FR, Goodenough DA, Paul DL. Landesman Y, Postma FR, Goodenough DA, Paul DL. Novel connexins contribute to intercellular communication in the Xenopus embryo. J Cell Sci 2003;116:29-38.

Goodenough DA, Paul DL. Beyond the gap: functions of unpaired connexons. Nat Rev Mol Cell Biol 2003;4:285-295.

Scherer SS, Paul DL. The connexin genes of myelinating glial cells. In: Lazzarini RA, Griffin J, Lassman H, Miller R, Armin-Nave K, Trapp B, editors. Myelin and its Diseases. Amsterdam: Elsevier Science; 2004, 599-608.

Kleopa KA, Orthmann JL, Enriquez A, Paul DL, Scherer SS. Unique distributions of the gap junction proteins connexin29, connexin32, and connexin47 in oligodendrocytes. Glia 2004, in press.

Altevogt BM, Paul DL. Four classes of intercellular channels between glial cells in the CNS. J Neurosci 2004, in press.

See publications for Dr. Paul via PubMed.

Contact Information

E-mail: David Paul, PhD
Professor of Neurobiology
Harvard Medical School/Children's Hospital