Normal atrial conduction requires similar abundances and homogeneous/overlapping distributions of two
connexins (Cx40 and
Cx43). The remodeling of myocyte connections and altered electrical conduction associated with
atrial fibrillation (AF) likely involves perturbations of these
connexins. We conducted a comprehensive series of experiments to examine the abundances and distributions of Cx40 and
Cx43 in the atria of AF patients. Atrial appendage tissues were obtained from patients with lone AF (paroxysmal or chronic) or normal controls.
Connexins were localized by double label immunofluorescence confocal microscopy, and their overlap was quantified.
Connexin proteins and mRNAs were quantified by immunoblotting and qRT-PCR. PCR amplified genomic
DNA was sequenced to screen for
connexin gene mutations or polymorphisms. Immunoblotting showed reductions of Cx40
protein (to 77% or 49% of control values in samples from patients with paroxysmal and chronic AF, respectively), but no significant changes of
Cx43 protein levels in samples from AF patients. The extent of
Cx43 immunostaining and its distribution relative to
N-cadherin were preserved in the AF patient samples. Although there was variability of Cx40 staining among paroxysmal AF patients, all had some fields with substantial Cx40 heterogeneity and reduced overlap with
Cx43. Cx40 immunostaining was severely reduced in all chronic AF patients. qRT-PCR showed no change in
Cx43 mRNA levels, but reductions in total Cx40
mRNA (to <50%) and Cx40 transcripts A (to ~50%) and B (to <25%) as compared to controls. No Cx40 coding region mutations were identified. The frequency of promoter polymorphisms did not differ between AF patient samples and controls. Our data suggest that reduced Cx40 levels and heterogeneity of its distribution (relative to
Cx43) are common in AF. Multiple mechanisms likely lead to reductions of functional Cx40 in atrial gap junctions and contribute to the pathogenesis of AF in different patients.