Human connexin disorders of the skin

Differentially altered Ca2+ regulation and Ca2+ permeability in Cx26 hemichannels formed by the A40V and G45E mutations that cause keratitis ichthyosis deafness syndrome

Mutations in GJB2, which encodes Cx26, are one of the most common causes of inherited deafness in humans. More than 100 mutations have been identified scattered throughout the Cx26 protein, most of which cause nonsyndromic sensorineural deafness. In a subset of mutations, deafness is accompanied by hyperkeratotic skin disorders, which are typically severe and sometimes fatal. Many of these syndromic deafness mutations localize to the amino-terminal and first extracellular loop (E1) domains. Here, we examined two such mutations, A40V and G45E, which are positioned near the TM1/E1 boundary and are associated with keratitis ichthyosis deafness (KID) syndrome. Both of these mutants have been reported to form hemichannels that open aberrantly, leading to “leaky” cell membranes. Here, we quantified the Ca²⁺ sensitivities and examined the biophysical properties of these mutants at macroscopic and single-channel levels. We find that A40V hemichannels show significantly impaired regulation by extracellular Ca²⁺, increasing the likelihood of aberrant hemichannel opening as previously suggested. However, G45E hemichannels show only modest impairment in regulation by Ca²⁺ and instead exhibit a substantial increase in permeability to Ca²⁺. Using cysteine substitution and examination of accessibility to thiol-modifying reagents, we demonstrate that G45, but not A40, is a pore-lining residue. Both mutants function as cell–cell channels. The data suggest that G45E and A40V are hemichannel gain-of-function mutants that produce similar phenotypes, but by different underlying mechanisms. A40V produces leaky hemichannels, whereas G45E provides a route for excessive entry of Ca²⁺. These aberrant properties, alone or in combination, can severely compromise cell integrity and lead to increased cell death.

Differentiation of Organotypic Epidermis in the Presence of Skin Disease-Linked Dominant-Negative Cx26 Mutants and Knockdown Cx26

In this study, we chose a differentiation-competent rat epidermal keratinocyte (REK) cell line to examine the role of Cx26 and disease-linked Cx26 mutants in organotypic epidermal differentiation. First, we generated stable REK cell lines expressing three skin disease-linked mutants (G59A, D66H and R75W). Second, we used an RNAi approach to knock down the expression of Cx26 in REKs. Interestingly, the three-dimensional (3D) architecture of the organotypic epidermis altered the intracellular spatial distribution of the mutants in comparison to 2D cultured REKs, highlighting the importance of using organotypic cultures. Unexpectedly, the presence of disease-linked mutants or the overexpression of wild-type Cx26 had little effect on the differentiation of the organotypic epidermis as determined by the architecture of the epidermis, expression of molecular markers indicative of epidermis differentiation (keratin 10, keratin 14, involucrin, loricrin) and stratification/cornification of the epidermis. Likewise, organotypic epidermis continued to differentiate normally upon Cx26 knockdown. While Cx26 has been reported to be upregulated during wound healing, no reduction in wound closure was observed in 2D REK cultures that expressed loss-of-function, dominant Cx26 mutants. In conclusion, we demonstrate that gain or loss of Cx26 function does not disrupt organotypic epidermal differentiation and offer insights into why patients harboring Cx26 mutations do not frequently present with more severe disease that encompasses thin skin.

Selective defects in channel permeability associated with Cx32 mutations causing X-linked Charcot-Marie-Tooth disease

The X-linked form of Charcot-Marie-Tooth disease (CMTX) is caused by mutations in connexin32 (Cx32), a gap junction protein expressed by Schwann cells where it forms reflexive channels that allow the passage of ions and signaling molecules across the myelin sheath. Although most mutations result in loss of function, several studies have reported that some retain the ability to form homotypic intercellular channels. To gain insight into the molecular defect of three functional CMTX variants, S26L, Delta111-116 and R220stop, we have used several fluorescent tracers of different size and ionic charge to compare their permeation properties to those of wild-type Cx32. Although all mutations allowed the passage of the dye with the smallest molecular mass, they exhibited a clear reduction in the permeability of either one or all of the probes with respect to wild-type channels, as assessed by the percentage of injections showing dye coupling. These data reveal that a lower size cutoff distinguishes these functional CMTX variants from wild-type channels and suggest that this defect may be of pathophysiological relevance.

Transport and function of cx26 mutants involved in skin and deafness disorders

We examined the subcellular localization and function of several Cx26 mutants that exhibit both sensorineural deafness and various skin disease phenotypes. To facilitate these aims, all Cx26 mutants were tagged at the carboxyl-terminal with green fluorescent protein (GFP), which has previously been shown not to affect Cx26 transport, assembly or function. In this article we focus on two point mutations (R75W and DeltaE42) that occur in the first extracellular loop region of Cx26, a region hypothesized to be critical for correct hemichannel docking between contacting cells. In gap junctional intercellular communication (GJIC)-deficient HeLa cells, both R75W-GFP and DeltaE42-GFP were transported to the cell surface and assembled into gap junction-like structures. Neither R75W-GFP nor DeltaE42-GFP formed gap junctions that were permeable to Lucifer Yellow suggesting they are loss-of-function mutations. We also examined the phenotype of these two mutations in a rat epidermal keratinocyte (REK) cell line that is capable of undergoing differentiation. Using antibodies against several members of the connexin family reportedly expressed by epidermal keratinocytes, we found these cells endogenously expressed Cx43 and Cx26 but not Cx30, Cx32, or Cx37. When expressed in REK cells, similar to in HeLa cells, R75W-GFP and DeltaE42-GFP were assembled at the cell surface into structures that resembled gap junctions. Future experiments will examine the effect of the Cx26 mutants on the function and differentiation of these epidermal keratinocytes.

Clouston Syndrome Can Mimic Pachyonychia Congenita

We studied three families suffering from nail abnormalities who had previously been diagnosed as pachyonychia congenita. No keratin gene mutations were detected. Sequencing of connexin 30 (GJB6 gene) in these patients identified heterozygous missense mutations G11R and A88V that are known to be associated with Clouston syndrome. This unexpected finding expands the Clouston syndrome phenotype and suggests that some patients diagnosed with pachyonychia may in fact be suffering from Clouston syndrome.

Divergent effects of two sequence variants of GJB3 (G12D and R32W) on the function of connexin 31 in vitro

Recently, we identified several missense mutations of the connexin gene GJB3 encoding connexin 31 (Cx31) in erythrokeratodermia variabilis (EKV), an autosomal dominant skin disorder. These mutations include G12D, which replaces a conserved glycine residue in the amino-terminus of Cx31 and is associated with a severe EKV phenotype. In contrast, the biologic relevance of the GJB3 sequence variant R32W located in the first transmembrane domain of Cx31 is disputed. To examine the effects of these sequence variants on Cx31 biogenesis and gap junction activity we expressed wild type and mutant Cx31-Flag constructs in HeLa cells. Using immunostaining, all expression variants were detected in the cytoplasm and in a punctate pattern at the cell surface, indicating that G12D and R32W did not interfere with either protein synthesis or transport to the cell membrane. Similarly, oligomerization into hemichannels appeared not impaired when expressing either Cx31 mutant as assessed by size exclusion chromatography, immunoblotting and immunostaining. However, dye transfer experiments and monitoring of intracellular calcium levels in response to serum stimulation revealed that G12D-Cx31 did not form functional gap junction channels, probably due to incorrect assembly or altered properties of Cx31 channels. In contrast, intercellular coupling between cells expressing R32W-Cx31 was comparable to that of wtCx31, suggesting that R32W is a functionally inconsequential polymorphism of Cx31.

Genetic disorders of palm skin and nail

The outer part of the skin, the epidermis, is specialized to protect the human body from its environment. Because of the high levels of physical stress experienced by the human hand in everyday use, the epidermis of the hand is especially toughened. In particular, the epidermis of the palm is highly specialized to resist mechanical trauma. Like the epidermis, the nails are composed of specialized epithelial cells and are especially strong. In recent years it has become apparent that the physical strength of epithelial cells comes from the keratin cytoskeleton--a dense meshwork of filaments extending throughout the cytoplasm. Keratins are a large family of intermediate filament proteins encoded by more than 50 distinct genes in humans. These different keratin genes are expressed in well-defined combinations in specific epithelial tissues. Several keratin genes are expressed in palmoplantar epidermis and in the stratified epithelia of the nail bed. Genetic mutations in these genes lead to fragility of these tissues and result in a range of genetic disorders characterized by blistering and thickening of palm and sole skin and/or nails. Study of these diseases has shed new light on the vital structural role of keratins in maintaining the integrity of epithelial cells.