Evolution of gap junction proteins the pannexin alternative
Institute of Problems of Information Transmission, Russian Academy of Science127994 Moscow, and A.N. Belozersky Institute of Physico-Chemical Biology, Moscow State University, Russia
(e-mail: ypanchin{at}yahoo.com)
Accepted 7 February 2005
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Summary |
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Key words: connexin, pannexin, gap junction, innexin, OPU
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Introduction |
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Animal specific intercellular channels are formed of proteins and are
called gap junctions (GJ). Physiological and morphological studies have
identified GJ in different tissues of various metazoan species. They all
appear to have similar physiological properties. Surprisingly it was found
that two unrelated protein families are involved in this function. Connexins
are found only in chordates. Pannexins (innexins) are present both in
invertebrate and chordate genomes (Baranova
et al., 2004; Bruzzone et al.,
1996
; Kumar and Gilula,
1996
; Levin, 2002
;
Panchin et al., 2000
;
Phelan et al., 1998a
;
Phelan and Starich, 2001
).
This article is a brief overview of current knowledge of the two families of gap junction proteins, with an emphasis on the pannexin family and the evolution of gap junction function.
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Initial gap junction studies |
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Before any GJ genes were identified and sequenced the GJ molecular
structure was predicted from X-ray diffraction and electron microscopy
(Caspar et al., 1977;
Makowski et al., 1977
). A
model was proposed in which a gap junction hemichannel is formed as six
subunits oligomerize to form a hexameric torus. The unit gap junction channel
is a pair of hemichannels, one from each cell, apposed in the narrow
intercellular gap between neighbouring cell membranes.
Although GJ are the most common intercellular channels in animals, and only
membrane-lined intercellular channels are known in plants and fungi,
membrane-coated pores can nevertheless be observed in certain animal cell
types (Huckins, 1978;
Rustom et al., 2004
;
Shestopalov and Bassnett,
2000
).
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Connexins |
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Pannexins |
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Both the human and the mouse genomes contain three pannexin-encoding genes.
The mammalian PANX1 (pannexin-1) mRNA is ubiquitously, although
disproportionately, present in different tissues; in the embryonic central
nervous system it is expressed noticeably more strongly than in other tissues.
PANX2 is a brain-specific gene. A low level of PANX3 was detected in the brain
and EST data suggest that PANX3 is expressed in osteoblasts and synovial
fibroblasts (Baranova et al.,
2004; Bruzzone et al.,
2003
; Panchin et al.,
2000
).
Direct proof of the vertebrate pannexins GJ function was provided by
Bruzzone and coworkers (Bruzzone et al.,
2003). They demonstrated that in paired oocytes, rodent PANX1,
alone and in combination with PANX2, induced the formation of intercellular
channels. However, it is not clear if pannexins duplicate GJ functions of
connexins in vertebrates or play some special physiological role
(Bao et al., 2004
).
Recently vinnexins (viral homologs of pannexins/innexins) were identified
in Polydnaviruses that occur in obligate symbiotic associations with
parasitoid wasps. It was suggested that virally encoded vinnexin proteins may
alter gap junctions in infected host cells, possibly affecting encapsulation
responses in parasitized insects (Kroemer
and Webb, 2004; Turnbull and
Webb, 2002
).
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Gap junction protein evolution |
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The growing number of cDNA and genomic sequences from different organisms
provide evidence that connexins are present in all vertebrates and also in
animals of the chordate branch, tunicates, ascidians and appendicularians,
(see Sasakura et al., 2003;
GenBank accession numbers AY380580, AY386312 and AY386311).
As recently as a few years ago there were no means of checking reliably
whether some genes are really absent in given genomes. This uncertainty has
changed with the availability of complete genome sequences from many model
organisms and allows us to assert that connexin and pannexin homologs are
absent in prokaryotes, plants and fungi
(Fig. 1E). This fact is
consistent with the hypothesis that multicellularity in plants, fungi and
animals emerged independently (Baldauf,
2003).
The most intriguing outcome of the survey of complete genomes for GJs was
the absence of the connexin homologs in non-chordate metazoan genomes such as
those nematode worms and fruit fly (The
C. elegans Sequencing Consortium, 1998;
Adams et al., 2000
).
The analysis of gene loss and acquisition is a powerful tool for
evolutionary studies (Koonin et al.,
2004). Specific genes for multicellularity are of particular
interest. Connexins appear to be chordate-specific genes
(Fig. 1E), but can this
assertion be substantiated? If connexins are present in the genome of other
deuterostomes, like echinoderms, we may expect connexins to arise from an
earlier deuterostome ancestor, and if they are found in basal radial
metazoans, such as Cnidaria, the plausible scenario will support the
hypothesis that the connexin gene(s) was lost in the non-chordate
(protostome?) common ancestor.
Unfortunately no complete genomes from non-chordate deuterostomes or radial
animals are available so this question cannot be resolved at present. We can
only state that in current databases no connexins are present outside the
Chordata. The pannexin story appears to be clearer. Pannexins are present in
all major bilaterian groups (Fig.
1E). Recently they were found in chordate branch tunicates
(Sasakura et al., 2003;
GenBank accession number TPA: BK005483). Apparent pannexins are also present
in hydra (Cnidaria) database sequences. From hydra ESTs in the GeneBank we
were able to reconstruct two complete coding sequences (CDS) of pannexin and
three more partial CDS of obvious pannexin orthologs (GenBank accession
numbers TPA: BK005478-BK005482). This finding strongly supports our postulate
that pannexins are ubiquitous metazoan proteins and further justifies their
name.
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Gap junction specificity and brain function |
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For both connexins and pannexins it was suggested that hemichannels in
opposing cells may have different subunit compositions and that this
difference can affect the assembly of the functional cellcell channel
(Ebihara et al., 1999;
He et al., 1999
;
Kelmanson et al., 2002
;
Starich et al., 1996
;
Stebbings et al., 2000
;
White et al., 1995
). The
diversity of gap junction molecules is high. Chordates encode about 20
different connexins and three pannexins in their genomes, whereas
invertebrates have about 20 different pannexins. Even the simple metazoan,
Hydra, has at least five pannexins and probably more. The number of
hexameric structures that can be produced by combinations of 20 monomers is
vast. There is growing evidence that hemichannel properties really depend on
the subunit composition. For the vertebrate GJ molecules (connexins), it was
shown that differential expression of two different connexins by two distinct
types of cells in mammalian heart is responsible for selective coupling
(White et al., 1995
). In
nematode and fly pannexins (innexins), mutations revealed defects in specific
GJ connections in the pharyngeal muscles and nervous system. For example,
eat-5 mutants lose detectable dye coupling, a reliable indicator of
GJ communication, between anterior and posterior pharyngeal muscle groups
(Starich et al., 1996
).
In molluscs the key role of pannexins in the process of GJ selection was
proved by intracellular injections of synthetic mRNA coding cPanx1, which led
to specific changes in the electrical connection patterns formed by injected
neurons (Kelmanson et al.,
2002).
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Conclusions |
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We conclude that pannexins are ubiquitous metazoan proteins, whereas connexins appear to be chordate specific. As chordates apparently possess two distinct types of gap junction molecules it is important to understand what is the balance between them. Do pannexins duplicate GJ functions of connexins or does each play its own physiological role? Very little data are available on this subject, especially data describing pannexin function.
GJ intercellular communication is peculiar because it is likely to require only one type of molecule to build a functional communication channel between two cells. It is suggested that the same GJ molecules that are responsible for channel formation are also mediating cellcell recognition, and that diversity of GJ proteins, together with the capability of forming heteromeric channels, provides the molecular basis for specificity of intercellular connections.
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Acknowledgments |
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