Emerging issues of connexin channels: biophysics fills the gap

Abstract

1. Introduction 326

1.1 What? Terminology and general properties 327

1.2 Why? Reasons for biophysical study 329

1.3 How? Special issues for study of connexin channels 330

2. Molecular and structural context 331

2.1 Biochemical features 331

2.2 Structures 334

2.2.1 Junctional channels 335

2.2.2 Hemichannels 338

2.2.3 Heteromeric channels 342

2.2.4 Junctional plaques 347

3. Experimental approaches and issues specific to study of connexin channel physiology 349

3.1 Macroscopic currents 349

3.1.1 Junctional channels 349

3.1.2 Hemichannels 354

3.2 Single-channel currents 355

3.2.1 Junctional channels 355

3.2.2 Hemichannels 358

3.3 Molecular permeability 361

3.3.1 A selection of tracers 361

3.3.2 Junctional channels 362

3.3.3 Hemichannels 366

3.4 Other 367

4. Structural issues 368

4.1 What lines the pore? 368

4.2 Docking between hemichannels 373

4.2.1 Structural and molecular basis 374

4.2.2 Determinants of specificity of interaction 380

5. Permeability and selectivity 381

5.1 Among the usual ions 383

5.1.1 Unitary conductance 383

5.1.2 Selectivity 384

5.1.3 Nonlinear single-channel I–V relations and their molecular determinants 386

5.2 Among large permeants 391

5.2.1 Uncharged molecules 392

5.2.2 Charged molecules 393

5.2.3 Cytoplasmic/signaling molecules 396

6. Voltage sensitivity 399

6.1 Macroscopic transjunctional voltage sensitivity 404

6.2 Microscopic voltage sensitivity – V_j-gating 407

6.2.1 Molecular basis – voltage sensor 407

6.2.2 Molecular basis – transduction and/or state stability 409

6.3 Microscopic voltage sensitivity – loop gating 412

6.4 V_m-gating 414

7. Direct chemical modulation 415

7.1 Phosphorylation 417

7.2 Cytoplasmic pH and aminosulfonates 419

7.3 Calcium ion 424

7.4 Lipophiles 424

7.4.1 Long chain n-alkyl alcohols 425

7.4.2 Fatty acids and fatty acid amides 426

7.4.3 Halothane 426

7.5 Glycyrrhetinic acid and derivatives 427

7.6 Cyclic nucleotides 428

7.7 Other candidates 430

8. Connexinopathies 431

9. Summary 435

10. Acknowledgements 438

11. References 438

Connexins are the proteins that form the intercellular channels that compose gap junctions in vertebrates. Connexin channels mediate electrotonic coupling between cells and serve important functions as mediators of intercellular molecular signaling. Convincing demonstration of the latter function has been elusive, as have the experimental tools required for detailed functional study of the channels. Recently, substantial progress has been made on both fronts. Connexin channels are now known to be dynamic, multifunctional channels intimately involved in development, physiology and pathology, and amenable to study by state-of-the-art approaches. A host of developmental and physiological defects are caused by defects in connexin channels, and therefore in the intercellular molecular movement they mediate. The channel structure has been determined to 7·5 Å resolution within the plane of the membrane. Experimental paradigms have been developed that enable application of the tools of modern channel biophysics to study connexin channel structure–function. As a result, the biophysical mechanisms and biological functions of connexin channels now enjoy a vigorous and expanding experimental interest. This article focuses on the former, but with attention to issues likely to have biological consequences.