SnapShot:SnapShot: XXXXXXXXXXXXXXXX Connexins and Disease 1 2 3 AUTHORDale W. XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX Laird, Christian C. Naus, and Paul D. Lampe 1Department of Anatomy and Cell Biology, University of Western Ontario, London, ON N6A 5C1, Canada AFFILIATION XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX 2Department of Cellular & Physiological Sciences, University of British Columbia, Vancouver, BC V6T 1Z3, Canada 3Fred Hutchinson Cancer Research Center, Seattle, WA 98109, USA Typical life cycle of connexins Mechanisms linked to connexin diseases Hyperactive mixed Hemichannel Loss of function Gain of function Leaky hemichannel hemichannel Dead hemichannel Gap junction Oligomer Lysosome Transdominant Defective channel Interactome Golgi retention turnover Monomer ERAD Aberrant Defective oligomerization interactome Connexosome Dead gap NUC LEUS junction channels Connexin Connexin Connexin Molecules subtype 1 subtype 2 interactor <1.5 kDa N U C LEUS Connexin subtype 1 Connexin subtype 2 Mutant connexin NUC LEUS Connexin genes linked to disease Diseases/genetic Principle Other connexin Inheritance Frequency Number of Most affected Other affected disorders connexin genes (protein) reported organ/tissue/cells organs/tissues gene (protein) mutations Sensorineural GJB2 (Cx26) GJB6 (Cx30), GJB3 (Cx31) Primarily recessive ~1-2/2000 ~135 Inner ear/cochlea Skin (epidermis) hearing loss Congenital cataracts GJA8 (Cx50) GJA3 (Cx46) Dominant and recessive 15% inherited cataracts ~70 Lens None reported Charcot-Marie-Tooth GJB1 (Cx32) None X-linked ~1/50,000 >300 PNS/Schwann cell Occasionally CNS disease x-linked myelination Pelizaeus-Merzbacher- GJC2 (Cx47) None Recessive Extremely rare ~40 CNS/oligodendro- Other CNS functions, like disease-1 cytes myelination Lymphatics Skin diseases GJB2 (Cx26) GJB6 (Cx30), GJB4 (Cx30.3), Primarily recessive Rare ~50 Skin (epidermis) Hearing GJB3 (Cx31), GJA1 (Cx43) Arrhythmias GJA5 (Cx40) GJA1 (Cx43) Dominant Extremely rare ~15 Heart Developing organs for GJA1 Oculodentodigital GJA1 (Cx43) None Primarily dominant ~1-2/100,000 ~75 Eyes, enamel, Bladder, skin, CNS, others dysplasia digits, facial bones Sensorineural Organ of Corti GJB2, Cx26 Skin diseases GJB2, Cx26 hearing loss Normal Hair cell death Connective Cornified Cochlea Epithelial gap tissue gap Granular junction network junction network Spinous Basal Mutation site Basement Congenital cataracts membrane GJA3, Cx46 GJA8, Cx50 Thicker Flaky and/or loss Eyee Lens Fiber cells of barrier function Cataract Epithelial cells Arrhythmias Normal Demyelinating diseases Pelizaeus- Merzbacher- GJC2, Cx47 Charcot-Marie-Tooth Astrocyte like disease-1 neuropathy x-linked Atriumm Atrial fibrillation Schwann cell Node of Ranvier Myelin sheath PNS CNS His bundle Branch bundles GJB1, Cx32 Purkinje fibers Oligodendrocyte Oculodentodigital dysplasia and related diseases Adult GJA5, Cx40 GJA1, Cx43 Ectoderm Small eyes Enamel loss Tooth decay Fused digits Zygote Day 2 Day 3 Morula Blastocyst Craniofacial bone embryo embryo Mesoderm Endoderm abnormalities Cx26 Cx30 Cx30.3 Cx31Cx32 Cx40 Cx43 Cx46 Cx47 Cx50 See online version for 1260 Cell 170, September 07, 2017 © 2017 Elsevier Inc. DOI http://dx.doi.org/10.1016/j.cell.2017.08.034 legends and references 2 Cell ???, ??MONTH?? ??DATE??, 200? ©200? Elsevier Inc. DOI XXXXXXXXX See online version for ??? SnapShot: Connexins and Disease Dale W. Laird,1 Christian C. Naus,2 and Paul D. Lampe3 1Department of Anatomy and Cell Biology, University of Western Ontario, London, ON N6A 5C1, Canada 2Department of Cellular & Physiological Sciences, University of British Columbia, Vancouver, BC V6T 1Z3, Canada 3Fred Hutchinson Cancer Research Center, Seattle, WA 98109, USA Connexin Life Cycle and Linked Diseases Gap junctions, composed of one or more of the 21-member connexin (Cx) family, are found in virtually every cell type of the human body. Their function as intercellular chan- nels is primarily defi ned by their ability to pass small signaling molecules, metabolites, and electrical stimuli directly between contacting cells. Given their ubiquitous distribution in the human body, it is not surprising that germ-line mutations in ten connexin genes lead to more than two dozen human diseases through loss- or gain-of-function molecular mechanisms (Srinivas et al., 2017; Kelly et al., 2015). Of note, the recognized role of connexins in human physiology and/or pathology has expanded to include hemichannel exchange of small molecules with the extracellular milieu (Laird, 2006). Connexins are integral membrane proteins that oligomerize into homomeric or heteromeric hexamers (also called connexons) in the endoplasmic reticulum or Golgi appara- tus. Upon microtubule-dependent transport to the cell surface, connexons may function as hemichannels or quickly dock with connexons from an opposing cell to assemble a gap junction channel. Connexons sequester into tightly packed structures termed gap junctions, which may be composed of a single or multiple connexin types. The connexin interactome that consists of dozens of binding proteins has been shown to play critical roles in gap junction assembly, stability, channel regulation, and intracellular signaling pathways. During turnover, one of the two contacting cells sharing a gap junction ingests an entire gap junction or gap junction fragments into a unique structure called a con- nexosome (or annular junction), where they fuse with lysosomes to be degraded. Most, but not all, connexins have an unusually short half-life of 1–4 hr (Laird, 2006). Contribution to Human Disease Disease-causing mutations in connexin genes have different effects on connexin proteins. In some cases, the mutant connexin fails quality control mechanisms and proceeds to endoplasmic reticulum associated degradation (ERAD) or gets stalled in the Golgi apparatus. Mutant connexins may lose the ability to form functional hemichannels or gap junction channels likely due to aberrant formation of the channel pore. In other cases, connexins acquire an aberrant half-life or lose interactions with the interactome, all of which can contribute to human pathologies. Gain-of-function mechanisms occur when some mutants bind or oligomerize with co-expressed connexins that they would normally not oligomerize with. These aberrant interactions can lead to transdominant effects on wild-type connexins that result in activated hemichannels and/or dead gap junction channels. Other connexin mutants form leaky homomeric hemichannels that may contribute to cellular pathologies. Impact on Sensory Organs. By far the most common connexin-linked disease is sensorineural hearing loss, which occurs in ~1/2,000 births (Chan and Chang, 2014). Mutation of the GJB2 gene encoding Cx26 constitutes the vast majority of children born with sensorineural hearing loss, with the 35delG mutation being the most common (Petit et al., 2001). The mechanism underpinning GJB2-linked hearing loss in the cochlea is still not fully understood but is likely linked to hair cell death due to aberrant metabolite or ion dispersion within the cochlea. Mutations in GJB3 (Cx31) and GJB6 (Cx30) have both also been linked to sensorineural hearing loss. The eyes are also impacted by mutations, as congenital cataract is the leading cause of reversible blindness in children. GJA3 (Cx46) and GJA8 (Cx50) are exceptionally long-lived connexins in the lens and function in this avascular tissue to enable metabolite, ion, and nutrient exchange (Srinivas et al., 2017). Demyelinating Diseases. GJB1 (Cx32) mutations disrupt the inter-lamellar cytoplasmic continuity within the Schmidt-Lanterman Incisures of Schwann cells, causing the demyelinating disorder Charcot-Marie-Tooth X-linked disease (CMTX1) (Bergoffen et al., 1993). Mutations in the GJC2 (Cx47) gene disrupt heterotypic gap junctions between astrocytes and oligodendrocytes within the CNS, which leads to impairments in oligodendrocyte function and the onset of an extremely rare demyelinating disease called Pelizaeus-Merzbacher-like disease-1 (aka- hypomyelinating leukodystrophy-2) (Uhlenberg et al., 2004). Skin Disease. Mutations in fi ve connexins genes (GJB2, GJB6, GJB4, GJB3, and GJA1) expressed in keratinocytes can cause many rare skin diseases that range in severity from mild increases in skin thickness to life-threatening and fatal barrier breakdown (Lilly et al., 2016). In the case of keratitis-ichthyosis with deafness, often dominant gain-of- function properties caused by leaky mutant connexin hemichannels act to dysregulate calcium homeostasis in the epidermis. Many mutants also cause hearing loss in patients. Others connexin-associated skin diseases include erythrokeratoderma (Cx30.3 and Cx31), Clouston syndrome (Cx30), and palmoplantar keratoderma and erythrokeratodermia variablilis et progressiva (Cx43). Arrhythmias. Given that the GJA5 (Cx40) gene is expressed in both the atria and the conduction system of the heart, it may not be surprising that mutations have been found in a few rare cases of early onset atrial fi brillation (AF) patients (Bai, 2014). One or more GJA1 (Cx43) gene mutations may also be associated with other arrhythmias (Lübkemeier et al., 2015). Oculodentodigital Dysplasia and Related Developmental Abnormalities. Oculodentodigital dysplasia (ODDD) is a complex disease involving multiple organs owing to the expression of the GJA1 (Cx43) gene early on during development and its ubiquitous distribution in the human body (Paznekas et al., 2009). ODDD patients typically exhibit small eyes, enamel