Division of anatomy

Faculty - William H. Fletcher, PhD

Research focus

The research focus of this group is on the physiologic roles and regulation of gap junction channels. In contrast to classic ion channels, gap junctions channels are conduits for the direct cell-to-cell exchange of small molecules, second messengers and morphogens that are involved in homeostasis or signaling pathways that coordinate the development and function of most tissues and organs.

For more than 10 years, we have been studying channels formed by the connexin43 protein, the product of 1 of 14 gap junction genes in mammals. Connexin43 is the dominant gap junction protein of the heart which produces at least three types of gap junctions during embryonic development. We approach our studies by using a diverse array of scientific disciplines, including the study of cell, molecular and developmental biology, biochemistry, and electrophysiology.

Previous studies have shown that connexin43 gap junction channels are rapidly and reversibly gated by phosphorylation events mediated by cAMP-dependent protein kinase and protein kinase C. Because of this and in view of the importance of gap junctions during development, we hypothesized that any connexin43 gene defect that altered a crucial site for regulation of connexin43 channels could result in abnormal formation or function of the heart. To test this, we took advantage of the fact that this campus is a dominant provider of pediatric heart transplants worldwide, and with a nearly 80% five-year survival rate for transplanted children, it received referral patients with the most severe types of heart defects.

Since beginning these studies in 1991, more than 150 patients have been evaluated for connexin43 gene defects. Of the more than 70 normal controls examined, no mutations have been found. Likewise, an equivalent number of transplant recipients have been examined, most of which also had only normal connexin43. However, 14 children with an extreme form of visceroatrial heterotaxia (VAH) in which complex heart defects are combine with visceral situs abnormalities had conexin43 mutations most of which replace serine residues that can be phosphorylated with amino acids that cannot be phosphorylated. This results in connexin43 channels that mediate communication to half the level of normal connexin43 and are also regulated incorrectly. Interestingly, the most common connexin43 mutation found in the VAH children also causes VAH when expressed in frog embryos.

Studies that are currently ongoing are all directed to examining the roles and regulation of gap junction channels in heart development. These include: biophysical properties of heart gap junction channels in response to protein kinase A and protein kinase C; continued analysis of the connexin43 gene in heart transplant children and their family members; development of a targeted insertion, i.e. "knock-in" strain of mice, that expresses the connexin43 mutations found in VAH patients; these will be used to follow heart development beginning with the earliest heart tube stage through to birth; molecular and biochemical manipulations of specific amino acids that are involved in the regulation of connexin43 channels.

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