Comparing the morphology and mechanisms of movement of a flagellated and an amoeboid cell. (A) Simplified depiction of the mammalian spermatozoon structure including the distribution of transporters/channels involved in the generation of Ca²⁺ domains necessary for sperm motility. The sperm cell consists of the head compartment (head subcompartments are acrosomal, equatorial, and postacrosomal segments), the midpiece, and the tail domain. At least three calcium storage locations are evident: (i) the acrosome, (ii) the redundant nuclear envelope region (RNE) and (iii) the mitochondria located in the midpiece. Ca²⁺ release from these stores is mediated by inositol 1,4,5‐tripohosphate receptors (IP3Rs) located on the outer acrosome membrane and by IP3Rs and RyRs in the RNE membrane. Moreover, extracellular Ca²⁺ can enter through a number of voltage‐ and ligand‐activated channels. Removal of Ca²⁺ occurs via plasmalemmal Na⁺/Ca²⁺ exchangers and, more rapidly, through various Ca²⁺‐ATPases (PMCA4, SPCA1, SERCA?) [modified, with permission, from Bedo‐Addu et al. (41)]. (B). Schematic overview of the subcompartments of a migrating cell and the relevant signaling pathways involved in cytoskeletal assembly. Rho contributes to the formation of stress fibers and focal adhesions, Rac to lamellipodial protrusion and focal complex formation, and Cdc42 to the development of filopodia and to the formation of focal complexes as well [modified, with permission, from Kaverina et al. (409)].

Functional morphology of the ciliary/flagellar axoneme. (A) Cross section of a typical ciliary or flagellar axoneme of the “9 + 2 structure” including its major components. The central pair of singlet microtubules, C1 and C2, is connected. C1 and C2 can be distinguished by fibrous structures attached to C1 only. The nine microtubule doublets consisting of an α and a β tubule are connected by nexin. The α tubule is decorated with the outer and inner dynein arms that mediate microtubule gliding and with radial spokes whose spokeheads point to the central microtubules. (B) Model of the sliding mechanism between outer doublet mictrotubules. A straight cilium shows the complete pattern (center of diagram). In a bent cilium, approximately half the filaments on the upper side are retracted because of the greater arc on the convex side. So the partial microtubules disappear being drawn below the plane of the slice. As seen here, bending to the left causes the partial microtubules 4, 5, 6, 7, and 8 to disappear. When the cilium bends the other way, the partial microtubules on the opposite side disappear while they reappear on what is now the lower or concave side. (C). In a flagellum as in the cilium, two adjacent doublets cannot slide far because (i) they are physically restrained by proteins (radial spokes and nexin links) and (ii) minus ends are usually anchored to the basal body, so they bend [A‐C were modified, with permission, from Warner and Satir (903) and Lodish et al. (492) according to Goodenough and Heuser (280)].

Comparison of sperm motility characteristics and activation pathways in mammals (left panel) and echinoderms (right panel). Mammalian spermatozoa are released into the bicarbonate containing millieu of the vagina (internal fertilization). Echinoderm spermatozoa are released into sea water—often synchronized with the release of eggs by female individuals (external fertilization). The bicarbonate (in mammals) and the speract (in echinoderms), a protein secreted from the egg's jelly coat, serve as activators of the signaling cascade resulting in directed motility. In both models, Ca²⁺ influx and intracellular alkaliniziation in response to cyclic nucleotide production and subsequent changes in the membrane potential play key roles in sperm bending. Mammalian spermatozoa exhibit hyperactivated motility upon Ca²⁺ influx when reaching the vicinity of the egg. Echinoderm sperm cells approach the egg in spirals with straight periods and turns. For further details see text. Abbreviations: sAC, soluble adenylyl cyclase; sNHE, sperm specific Na⁺/H⁺ exchanger; KSper, sperm‐associated potassium channel; CatSper, sperm‐associated cation channel; CM, calmodulin; CMK, calmodulin kinase; TetraKCNG, cGMP‐regulated potassium channel; NCKX, potassium‐dependent Na⁺/Ca²⁺ exchanger; SpHCN, sperm hyperpolarization‐activated and cyclic nucleotide‐gated channel; Ca_v, voltage‐gated Ca²⁺ channel [modified, with permission, after Navarro et al. (581) and Darszon et al. (161)].

Structural basis for cell adhesion and migration. (A) Model of actin assembly at the leading edge of the lamellipodium (shaded area) of migrating cells. (i) While being in the resting state, the barbed ends of actin filaments are capped by capping proteins. Upon directing stimuli cells fade from the resting state to a state of morphological and functional polarization. At the leading edge of the lamellipodium free barbed ends are generated by the dissociation of capping proteins. (ii) The activated Arp2/3 complex accounts for the assembly of Y‐junctions. (iii) Active Arp2/3 complex mediates the connection of preexisting filaments. Branched filaments that mainly consist of adenosine triphosphate (ATP)‐ and adenosine diphosphate (ADP)‐P_i‐actin grow fast and represent the fundament of protrusion shape and stability. (iv) Branched filaments are depolymerized at the rear of the lamellipodium by severing or removal of Arp2/3 complexes. (v) Barbed ends of filaments are recapped to prevent further filament growth. (vi) ADF (actin depolymerizing factor)‐cofilin/ADP‐actin complexes and monomers in equilibrium are the intermediate product of filament dissociation. (vii) Transfer of phosphate from ATP to ADP‐actin enhanced by profilin. (viii) ATP‐actin monomers that are prevented from spontaneous nucleation by β‐thymosin are now available for reassembly [adopted, with permission, from Gungabissoon and Bamburg (306)]. (B).Schematic overview of the dynamics of adhesion complexes (AC) and morphological features of a migrating cell. Cell migration requires the continuous formation and release of cell matrix interactions accompanied by a protruding movement at the front of the cell and a retracting movement at the cell rear. Characteristic features of the cell protrusion (shaded area) are the fan‐shaped lamellipodium and finger‐like filopodia, while inside the rear part the cell develops retraction fibers. Adhesion complexes are initially generated at cell protrusions as focal complexes (small red dots) and mature to larger focal adhesions (elongated red ovals) that slide backward toward the rear part where they are finally disassembled. Numbers within circles represent different states of AC dynamics. (i) Priming of AC: Upon external signals the cell polarizes and structures such as the lamellipodium and filopodia are formed involving the polymerization of actin filaments. The adhesion of these protrusions to components of the extracellular matrix (ECM) requires integrin recruitment to transform these sites into “sticky fingers.” (ii) Initiation of AC: The initiation of AC requires the formation of a branched F‐actin meshwork inside the lamellipodia as the fundament for focal complexes. The F‐actin meshwork inside the cell protrusion is induced by the activation of the small GTPase Rac1. F‐actin polymerization is facilitated by the activation of the Arp2/3 complex (see Figure 4A). Cofilin creates free barbed ends by severing the filaments. To become parts of this meshwork integrins have to be incorporated. They cluster and then serve as anchors tying the cell to immobile components of the ECM. Talin is crucial for adapting the actin filaments to integrins at adhesion sites. The newly generated small focal complexes are characterized by a slow integrin turnover and, thus, a high stability supporting their relatively immobilized state. Furthermore, additional structural and signaling components translocate to focal complexes which now serve as signaling platforms to perpetuate lamellipodia formation. (iii) Maturation of AC: The conversion of small focal complexes into larger focal adhesions with high integrin density is referred to as maturation of AC. The RhoA‐dependent generation of stress fibers is a characteristic feature of the cytoskeleton of migrating cells. Tensioning of stress fibers stabilizes the polarized cell structure as well as the lamellipodium. The mission of AC as an anchor tying the cytoskeleton to the ECM is to sense, transmit and respond to intra‐ or extracellular tension. At the level of sensing, enzymatic reactions as well as tension sensitivity of adaptor proteins are discussed to be involved. Adaptor proteins such as zyxin or the ILK‐PINCH‐parvin complex serve as mediators at the level of signaling to the nucleus. (iv) Sliding of AC: Functional components of AC move as consequence of and in the direction of stress fiber tensioning. The sliding of mature AC is characterized by a polarized turnover of AC components such as the integrins. Integrin molecules are assembled at the cell front and disassembled at the cell rear, whereas other molecules undergo a constant exchange. Acto‐myosin tensioning plays a strong role in the control of local AC dynamics. (v) Disassembly of AC: The disassembly of AC is critical to the overall speed of migrating cells and does not exclusively occur at the rear part. Inhibition of AC disassembly inhibits cell migration. AC disassembly is characterized by integrin internalization. Mechanisms that promote AC turnover may also contribute to their disassambly such as RhoA and myosin contractility. Moreover, the cleavage of talin by calpain may be essential for disassembly processes as well as dynamin and focal adhesion kinase (FAK)‐mediated vesicular trafficking [modified, with permission, after Lock et al. (491)].

The role of the transepithelial potential in galvanotactic wound healing. (A) Molecular basis of the electric potential difference of the cornea epithelium. Upper panel: net flux of Cl⁻ ions from the basolateral side toward the apical side and Na⁺ flux in the opposite direction generates an electric potential difference. Lower panel: ion channels and transporters that are involved in transcellular ion transport (thin arrows) and paracellular ion flux through tight junctions (thick arrows). cAMP‐activated chloride channels, Ca²⁺‐dependent chloride channels, epithelial sodium channels, and sodium/org cotransporters are involved at the apical membrane. Sodium‐chloride‐potassium cotransporters, sodium/potassium ATPases, potassium channels, and sodium‐potassium/proton exchangers are involved at the basolateral membrane [adopted, with permission, from Zhao (957)]. (B) Generation of wound electric fields. Upon physical disruption the transepithelial potential (see Figure 5 A) is short‐circuited and becomes negative. To maintain the transepithelial potential, cells surrounding the wound fuel a positive charge flow toward the wounded area (red arrow) and consequently out of the wound (black arrows) until the wound is healed [adopted, with permission, from Zhao (957)]. (C) Schematic representation of possible mechanisms that could facilitate galvanotaxis mediated by voltage‐gated sodium channels (VGSC). Upon voltage‐driven influx of Na⁺ through VGSC several mechanisms may lead to a Ca²⁺‐dependent reorganization of the cytoskeleton and eventually to directional cell migration. VGSC's β‐subunit is assumed to directly interact with the cytoskeleton. Intracellularly elevated Na⁺ levels are likely to inhibit or promote other ion‐transport processes finally resulting in an elevation of the intracellular Ca²⁺ concentration: due to a decrease in the Na⁺ gradient across the plasma membrane, Ca²⁺ removal from the cytosol via the Na⁺/Ca²⁺ exchanger is reduced; pH‐regulating mechanisms such as Na⁺/H⁺ exchange are inhibited and the cytosol becomes acidic. The increase in the cytosolic H⁺ concentration then (i) lowers cytosolic Ca²⁺ removal through the mitochondrial Ca²⁺/H⁺ exchanger, (ii) lowers ATPase‐mediated Ca²⁺ uptake into the endoplasmic reticulum (ER), and (iii) triggers Ca²⁺ release from the ER via IP3 and ryanodin receptors [modified, with permission, after Mycielska and Djamgoz (573)].

The role of the transepithelial potential in galvanotactic wound healing. (A) Molecular basis of the electric potential difference of the cornea epithelium. Upper panel: net flux of Cl⁻ ions from the basolateral side toward the apical side and Na⁺ flux in the opposite direction generates an electric potential difference. Lower panel: ion channels and transporters that are involved in transcellular ion transport (thin arrows) and paracellular ion flux through tight junctions (thick arrows). cAMP‐activated chloride channels, Ca²⁺‐dependent chloride channels, epithelial sodium channels, and sodium/org cotransporters are involved at the apical membrane. Sodium‐chloride‐potassium cotransporters, sodium/potassium ATPases, potassium channels, and sodium‐potassium/proton exchangers are involved at the basolateral membrane [adopted, with permission, from Zhao (957)]. (B) Generation of wound electric fields. Upon physical disruption the transepithelial potential (see Figure 5 A) is short‐circuited and becomes negative. To maintain the transepithelial potential, cells surrounding the wound fuel a positive charge flow toward the wounded area (red arrow) and consequently out of the wound (black arrows) until the wound is healed [adopted, with permission, from Zhao (957)]. (C) Schematic representation of possible mechanisms that could facilitate galvanotaxis mediated by voltage‐gated sodium channels (VGSC). Upon voltage‐driven influx of Na⁺ through VGSC several mechanisms may lead to a Ca²⁺‐dependent reorganization of the cytoskeleton and eventually to directional cell migration. VGSC's β‐subunit is assumed to directly interact with the cytoskeleton. Intracellularly elevated Na⁺ levels are likely to inhibit or promote other ion‐transport processes finally resulting in an elevation of the intracellular Ca²⁺ concentration: due to a decrease in the Na⁺ gradient across the plasma membrane, Ca²⁺ removal from the cytosol via the Na⁺/Ca²⁺ exchanger is reduced; pH‐regulating mechanisms such as Na⁺/H⁺ exchange are inhibited and the cytosol becomes acidic. The increase in the cytosolic H⁺ concentration then (i) lowers cytosolic Ca²⁺ removal through the mitochondrial Ca²⁺/H⁺ exchanger, (ii) lowers ATPase‐mediated Ca²⁺ uptake into the endoplasmic reticulum (ER), and (iii) triggers Ca²⁺ release from the ER via IP3 and ryanodin receptors [modified, with permission, after Mycielska and Djamgoz (573)].

Hypothetical model summarizing the local function of ion channels and transporters in migrating cells. At the leading edge salt uptake mediated by the Na⁺/H⁺ exchanger (NHE1), the Cl⁻/HCO₃⁻ exchanger (AE2), and the Na⁺,K⁺,2Cl⁻ cotransporter (NKCC) is accompanied by osmotic water entry. The water entry is facilitated by the aquaporin AQP1 and contributes to the extension of the lamellipodium. Toward the rear end an increase in membrane tension activates mechanosensitive cation channels resulting in an increase of the intracellular Ca²⁺ concentration. This rise in intracellular Ca²⁺ induces the retraction of the rear part of a migrating cell and a massive K⁺ efflux through Ca²⁺‐sensitive K⁺ channels accompanied by shrinkage of the posterior cell pole [modified, with permission, from Schwab (751,752)].

Hypothetical model summarizing how pH‐regulating and proton‐sensitive (transport) molecules modulate pH‐dependent intra‐ and extracellular processes required for cell migration. (A) Transporters and mechanisms involved in (i) regulating pH_i and (ii) in generating a characteristic pH profile at the cell surface. The functional cooperation between AE2, CA IX, and Na⁺,HCO₃⁻ cotransporter (NBC) could maximize the HCO₃⁻‐gradient across the membrane and thus optimize the buffering of pH_i. (B) Effects of pH_i on the cellular migration machinery. The bond between actin and talin is weakened by an alkaline pH_i at the cell front (i) and stabilized by an acidic pH_i at the rear end (ii). Cofilin is activated at an alkaline pH_i and produces free barbed‐end actin required for actin branching (iii), that is, for pushing the leading edge forward. An acidic pH_i inactivates cofilin (iv). It also promotes myosin II light chain phosphorylation by Ca²⁺‐calmodulin which causes actomyosin contraction at the rear part of the cell (v). (C) Effects of pH_e on migrating tumor cells. At the cell front, formation and stabilization of integrin/matrix interactions (i) and activity of matrix digesting MMPs (ii) are promoted by an acidic pH_e. At the cell rear, the higher pH_e facilitates the release of focal adhesions (iii). Activity of TRPM7 channels depends on pH_e: an acidic pH_e induces inward currents carried by monovalent cations possibly entailing local osmotic swelling that could be facilitated by the presence of aquaporins (AQP1). At the same time, TRPM7 executes α‐kinase activity phosphorylating the myosin IIA heavy chain which causes the disassembly of myosin bundles (iv). An alkaline pH_e at the cell rear increases TRPM7's selectivity for Ca²⁺ leading to (i) an increase in contractility (v) triggered by the Ca²⁺‐induced activation of the calmodulin‐modulated myosin II regulatory light chain (vi) and (ii) to the disassembly of focal adhesion sites mediated by m‐calpain (vii). ASIC responds to an acidic pH_e by mediating Na⁺ inward currents (viii), ovarian cancer G‐protein‐coupled receptor 1 (OGR1) activates intracellular signaling cascades (ix) [adopted, with permission, from Stock and Schwab (813)].

Major K⁺ and Ca²⁺ channels involved in cell migration. For the sake of clarity, this drawing does not include all of the Ca²⁺ and K⁺ channels mentioned in the text. K_v1.3 colocalizes with β1 integrin (23). While K_v1.3 can be evenly distributed all over a migrating cell (189), its clustering with transient receptor potential channel 1 (TRPC1) channels is found at the leading edge and probably involved in electric field detection (419). K_v2.1 shows a fibronectin‐dependent polarized distribution at the leading edge and the trailing end (417). While fibronectin stimulates the interaction between K_v2.1 and focal adhesion kinase (FAK) at the leading edge (910), probably via α8β1 integrin and activated by Ca²⁺/calmodulin‐dependent protein kinase (CaMKII) (220), the Ca²⁺‐sensitive calpain2 (m‐calpain), believed to be membrane bound, functions at the trailing edge of the migrating cell to cleave the integrins. Also stimulated by fibronectin, K_v11.1 (hERG1) channels and β1 integrins form a macromolecular complex (132). Once engaged by the proper ligand, integrins can activate K_v11.1 channels which, in turn, modulate integrin function (651). K_ir4.2 and α9 integrin colocalize at focal adhesions of the leading edge where the α9 integrin subunit simulates cell migration by a localized polyamine (spermidine/spermin=Sper) catabolism (168): α9 integrin binds the spermidine/spermin acetyltransferase (SSAT). SSAT activity catabolizes the polyamines that otherwise would block K⁺ ion efflux through K_ir4.2. Even though a polarized distribution of K_Ca1.1 has not been shown to date, its activity impedes migration of glioma cells (65,439) whereas in fibroblast –like synoviocytes it is needed for invasion and for the production of pro‐MMP‐2 (358). K_Ca1.1 can be activated by Ca²⁺ influx though TRPM8 channels (921,922). K_Ca 3.1 recycles to the leading edge (757), however, is mainly active at the cell body and the cell rear (694) upon Ca²⁺ entry (762) through stretch activated channels such as TRPM7 (128,816). TRPM7‐mediated Ca²⁺ influx contributes to the guidance of the leading edge (667,908,909), for example, toward a chemoattractant. At lateral and peripheral adhesions, activation of TRPM7 by stretch or by Mg‐ATP depletion causes Ca²⁺ influx that can be enhanced by ryanodine receptor (RyR) mediated Ca²⁺ release from the endoplasmic reticulum (ER). This local increase in [Ca²⁺]_i promotes cell migration through m‐calpain‐mediated disassembly of focal adhesions (816) and possibly a stimulation of the cytoskeletal migration machinery. At the trailing end, Ca²⁺‐dependent phosphorylation of contractile proteins is mediated by Ca²⁺ influx through L‐type voltage‐gated Ca²⁺ channels [VGCCs (943)], while TRPV1 (899), TRPM8, and TRPC1 (359) enhance cell migration by still unknown mediators. IP₃R‐ and RyR‐mediated Ca²⁺ mobilization from the ER generally promotes cell migration. Focal adhesion formation and turnover is facilitated by IP₃R‐mediated Ca²⁺ release stimulated through G‐protein coupled receptors causing CaMKII‐dependent FAK phosphorylation (220) and increased actin assembly (not shown) and by stromal interaction molecule 1 (STIM1)‐calcium release‐activated calcium channel protein 1 (ORAI1)‐based store‐operated Ca²⁺ entry leading to RAS and RAC activation (944), respectively. Ca²⁺ influx through TRPV2, recruited to the plasma membrane in response to simulation with lysophosphatidylcholine or lysophosphatidylinositol (LPL), induces matrix‐metalloproteases MMP‐2, 9, and cathepsin B (561,667). For further details, please see text and Tables 1 and 3.

Cellular Cl⁻ transport. Cl⁻ ions are passively and actively transported across cellular membranes. Passive flux of Cl⁻ is facilitated by a variety of channels including Ca²⁺‐activated Cl⁻ channels (CaCC), cell volume‐regulated anion channels (VRAC), voltage‐gated Cl⁻ channels (VGClC), ligand‐gated anion channels (LGAC), and cAMP‐activated Cl⁻ channels (CFTR). Several proteins facilitate active Cl⁻ transport into the cell (Cl⁻ loaders) or pump Cl⁻ ions out of the cell (Cl⁻ extruders). Cl⁻ loaders include Na⁺, K⁺, Cl⁻ cotransporters (NKCC), Cl⁻/HCO₃⁻ (AE), and Na⁺, Cl⁻ cotransporters (NCC). Cl⁻ extruders include K⁺, Cl⁻ cotransporters (KCC) and the Na⁺‐dependent Cl⁻/HCO₃⁻ exchanger (NDCBE). Furthermore, Cl⁻ channels and transporters play a role in vesicular pH and Cl⁻ homeostasis that is essential to vesicular trafficking. Blue arrows represent Cl⁻ transport [modified, with permission, after Duran et al. (200)].

Schematic overview of the role of Cl⁻, K⁺, and water influx in the lamellipodium formation of migrating cells. In contrast to swelling‐activated Cl⁻ channels and aquaporins that are evenly distributed in morphologically nonpolarized cells, K⁺, Cl⁻ cotransporters are concentrated at one “pole” of a migrating cell. Upon exposure to a hypoosmotic solution cells swell equally as water and Cl⁻ flux occurs all over of the cell (left panel). Superfusing the cells with KCl provokes lamellipodium formation by reversing the flux direction of K⁺ and Cl⁻ ions from outward to inward (K⁺, Cl⁻ cotransporter). Local increases in K⁺ and Cl⁻ concentrations are accompanied by a locally increasing osmolarity that induces water influx via aquaporins and, thus, results in unilateral swelling [modified, with permission, after Zierler et al. (966)].