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Relating structure and function of inner hair cell ribbon synapses.

Wichmann C, Moser T - Cell Tissue Res. (2015)

Bottom Line: Accumulating evidence indicates a highly specialized molecular composition and structure of the presynapse, adapted to suit these high functional demands.Relating structure and function has become an important avenue in addressing these points and has been applied to normal and genetically manipulated hair cell synapses.Here, we review some of the exciting new insights gained from recent studies of the molecular anatomy and physiology of IHC ribbon synapses.

View Article: PubMed Central - PubMed

Affiliation: Molecular Architecture of Synapses Group, Institute for Auditory Neuroscience and InnerEarLab, University Medical Center Göttingen, Göttingen, Germany, cwichma@gwdg.de.

ABSTRACT
In the mammalian cochlea, sound is encoded at synapses between inner hair cells (IHCs) and type I spiral ganglion neurons (SGNs). Each SGN receives input from a single IHC ribbon-type active zone (AZ) and yet SGNs indefatigably spike up to hundreds of Hz to encode acoustic stimuli with submillisecond precision. Accumulating evidence indicates a highly specialized molecular composition and structure of the presynapse, adapted to suit these high functional demands. However, we are only beginning to understand key features such as stimulus-secretion coupling, exocytosis mechanisms, exo-endocytosis coupling, modes of endocytosis and vesicle reformation, as well as replenishment of the readily releasable pool. Relating structure and function has become an important avenue in addressing these points and has been applied to normal and genetically manipulated hair cell synapses. Here, we review some of the exciting new insights gained from recent studies of the molecular anatomy and physiology of IHC ribbon synapses.

No MeSH data available.


Related in: MedlinePlus

Endocytosis in inner hair cells. a, a’ Representative recordings in response to 20 ms (a) or 200 ms (a’) depolarizations. After the Cm increase upon 20 ms depolarization, the slope-corrected Cm traces (middle) typically showed a linear decay (a). The 200-ms-long depolarization resulted in a combination of exponential and linear decay (a’). Modified from Neef et al. (2014) reprinted with permission from © 2014 Neef et al. b–b”’ 3D reconstructions of resting (b), stimulated (b’) and recovered IHCs (b”, b”’). Endocytotic organelles are shown in purple. Note the presence of tubular organelles both before and after stimulation. Most organelles, including the tubular ones, are replaced by small vesicles during the recovery periods. Insets magnified regions from the four different cell regions (cuticular plate, top, nuclear and basal regions). Note the increased number of endosome-like organelles at the base of the cell after stimulation and during recovery. Modified from Kamin et al. (2014), reprinted with permission © 2014 Kamin et al. c mCLING-labeled organs of Corti were immunostained for Vglut3 and otoferlin (first row), for Vglut3 and syntaxin 6 (Sx 6, second row), for otoferlin and syntaxin 16 (Sx 16, third row) and finally for syntaxin 6 and syntaxin 16 (fourth row). The samples were cut into 20-nm sections and were imaged using an epifluorescence microscope. Dashed white lines the plasma membrane of the IHCs. White arrowheads organelles where the signals for mCLING and the two immunostained proteins colocalized. Scale bar 2 μm. d Graphic representation of Pearson’s correlation coefficients: otoferlin and syntaxin 6 (or syntaxin 16) correlate in the mCLING-labeled organelles at the top and nuclear levels. Vglut3 correlates best with otoferlin at the basal level. At least 100 organelles were analyzed for each condition. Error bars SEMs. e Model of membrane recycling in IHCs. Organelles with a different molecular composition recycle membrane in different regions, taking up mCLING. Apical endocytosis takes up the membrane into round organelles, a sizeable proportion of which is similar to late endosomes (light blue). Endocytosis in the top and nuclear regions reaches tubular organelles containing otoferlin and two endosome markers, syntaxin 16 and syntaxin 6. This suggests that these organelles participate in constitutive pathways, probably by maintaining membrane traffic between the plasma membrane and the trans-Golgi. At the base of the cell, stimulation induces the formation of membrane infoldings and cisterns that are characterized by the presence of Vglut3, Rab3 and also otoferlin. (c–e modified from Revelo et al. 2014, reprinted with permission from © 2014 Revelo et al.)
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Fig3: Endocytosis in inner hair cells. a, a’ Representative recordings in response to 20 ms (a) or 200 ms (a’) depolarizations. After the Cm increase upon 20 ms depolarization, the slope-corrected Cm traces (middle) typically showed a linear decay (a). The 200-ms-long depolarization resulted in a combination of exponential and linear decay (a’). Modified from Neef et al. (2014) reprinted with permission from © 2014 Neef et al. b–b”’ 3D reconstructions of resting (b), stimulated (b’) and recovered IHCs (b”, b”’). Endocytotic organelles are shown in purple. Note the presence of tubular organelles both before and after stimulation. Most organelles, including the tubular ones, are replaced by small vesicles during the recovery periods. Insets magnified regions from the four different cell regions (cuticular plate, top, nuclear and basal regions). Note the increased number of endosome-like organelles at the base of the cell after stimulation and during recovery. Modified from Kamin et al. (2014), reprinted with permission © 2014 Kamin et al. c mCLING-labeled organs of Corti were immunostained for Vglut3 and otoferlin (first row), for Vglut3 and syntaxin 6 (Sx 6, second row), for otoferlin and syntaxin 16 (Sx 16, third row) and finally for syntaxin 6 and syntaxin 16 (fourth row). The samples were cut into 20-nm sections and were imaged using an epifluorescence microscope. Dashed white lines the plasma membrane of the IHCs. White arrowheads organelles where the signals for mCLING and the two immunostained proteins colocalized. Scale bar 2 μm. d Graphic representation of Pearson’s correlation coefficients: otoferlin and syntaxin 6 (or syntaxin 16) correlate in the mCLING-labeled organelles at the top and nuclear levels. Vglut3 correlates best with otoferlin at the basal level. At least 100 organelles were analyzed for each condition. Error bars SEMs. e Model of membrane recycling in IHCs. Organelles with a different molecular composition recycle membrane in different regions, taking up mCLING. Apical endocytosis takes up the membrane into round organelles, a sizeable proportion of which is similar to late endosomes (light blue). Endocytosis in the top and nuclear regions reaches tubular organelles containing otoferlin and two endosome markers, syntaxin 16 and syntaxin 6. This suggests that these organelles participate in constitutive pathways, probably by maintaining membrane traffic between the plasma membrane and the trans-Golgi. At the base of the cell, stimulation induces the formation of membrane infoldings and cisterns that are characterized by the presence of Vglut3, Rab3 and also otoferlin. (c–e modified from Revelo et al. 2014, reprinted with permission from © 2014 Revelo et al.)

Mentions: Currently, in IHCs, three distinct mechanisms are considered to mediate endocytosis: slow CME, fast bulk endocytosis and potentially kiss-and-run or ‘ultrafast’ endocytosis (Neef et al. 2014). CME is the main pathway of membrane retrieval for mild stimulation and proceeds at a constant rate; it represents the linear component of endocytosis following exocytosis of the RRP (Fig. 3a). This mechanism is not only inhibited by the clathrin-inhibitor pitstop-2 but also by disruption of dynamin 1 via pharmacological and genetic means (Neef et al. 2014). None of these manipulations seem to affect exocytosis. In contrast, a different study reported inhibition of sustained exocytosis by the presumptive dynamin inhibitor dynasore but did not investigate endocytic membrane retrieval (Duncker et al. 2013). Finally, when exocytosis exceeds three to four RRP equivalents, IHCs additionally recruit a faster mode of membrane retrieval, which proceeds with an exponential time course within a few seconds. It has been proposed to represent bulk endocytosis (Neef et al. 2014; Fig. 3a’) and, indeed, there is plenty of evidence for the invagination and fission of large stretches of plasma membrane in the vicinity of hair cell AZs (Lenzi et al. 2002; Frank et al. 2010; Pangršič et al. 2010; Kamin et al. 2014; Neef et al. 2014; Revelo et al. 2014). Both mechanisms seem to engage in different phases of release: CME supports vesicle cycling during mild stimulation but bulk endocytosis finally occurs after prolonged stimulation, providing a mechanism that assures the balance between exo- and endocytosis in IHCs and thus, assures high release rates (Neef et al. 2014).Fig. 3


Relating structure and function of inner hair cell ribbon synapses.

Wichmann C, Moser T - Cell Tissue Res. (2015)

Endocytosis in inner hair cells. a, a’ Representative recordings in response to 20 ms (a) or 200 ms (a’) depolarizations. After the Cm increase upon 20 ms depolarization, the slope-corrected Cm traces (middle) typically showed a linear decay (a). The 200-ms-long depolarization resulted in a combination of exponential and linear decay (a’). Modified from Neef et al. (2014) reprinted with permission from © 2014 Neef et al. b–b”’ 3D reconstructions of resting (b), stimulated (b’) and recovered IHCs (b”, b”’). Endocytotic organelles are shown in purple. Note the presence of tubular organelles both before and after stimulation. Most organelles, including the tubular ones, are replaced by small vesicles during the recovery periods. Insets magnified regions from the four different cell regions (cuticular plate, top, nuclear and basal regions). Note the increased number of endosome-like organelles at the base of the cell after stimulation and during recovery. Modified from Kamin et al. (2014), reprinted with permission © 2014 Kamin et al. c mCLING-labeled organs of Corti were immunostained for Vglut3 and otoferlin (first row), for Vglut3 and syntaxin 6 (Sx 6, second row), for otoferlin and syntaxin 16 (Sx 16, third row) and finally for syntaxin 6 and syntaxin 16 (fourth row). The samples were cut into 20-nm sections and were imaged using an epifluorescence microscope. Dashed white lines the plasma membrane of the IHCs. White arrowheads organelles where the signals for mCLING and the two immunostained proteins colocalized. Scale bar 2 μm. d Graphic representation of Pearson’s correlation coefficients: otoferlin and syntaxin 6 (or syntaxin 16) correlate in the mCLING-labeled organelles at the top and nuclear levels. Vglut3 correlates best with otoferlin at the basal level. At least 100 organelles were analyzed for each condition. Error bars SEMs. e Model of membrane recycling in IHCs. Organelles with a different molecular composition recycle membrane in different regions, taking up mCLING. Apical endocytosis takes up the membrane into round organelles, a sizeable proportion of which is similar to late endosomes (light blue). Endocytosis in the top and nuclear regions reaches tubular organelles containing otoferlin and two endosome markers, syntaxin 16 and syntaxin 6. This suggests that these organelles participate in constitutive pathways, probably by maintaining membrane traffic between the plasma membrane and the trans-Golgi. At the base of the cell, stimulation induces the formation of membrane infoldings and cisterns that are characterized by the presence of Vglut3, Rab3 and also otoferlin. (c–e modified from Revelo et al. 2014, reprinted with permission from © 2014 Revelo et al.)
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

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Fig3: Endocytosis in inner hair cells. a, a’ Representative recordings in response to 20 ms (a) or 200 ms (a’) depolarizations. After the Cm increase upon 20 ms depolarization, the slope-corrected Cm traces (middle) typically showed a linear decay (a). The 200-ms-long depolarization resulted in a combination of exponential and linear decay (a’). Modified from Neef et al. (2014) reprinted with permission from © 2014 Neef et al. b–b”’ 3D reconstructions of resting (b), stimulated (b’) and recovered IHCs (b”, b”’). Endocytotic organelles are shown in purple. Note the presence of tubular organelles both before and after stimulation. Most organelles, including the tubular ones, are replaced by small vesicles during the recovery periods. Insets magnified regions from the four different cell regions (cuticular plate, top, nuclear and basal regions). Note the increased number of endosome-like organelles at the base of the cell after stimulation and during recovery. Modified from Kamin et al. (2014), reprinted with permission © 2014 Kamin et al. c mCLING-labeled organs of Corti were immunostained for Vglut3 and otoferlin (first row), for Vglut3 and syntaxin 6 (Sx 6, second row), for otoferlin and syntaxin 16 (Sx 16, third row) and finally for syntaxin 6 and syntaxin 16 (fourth row). The samples were cut into 20-nm sections and were imaged using an epifluorescence microscope. Dashed white lines the plasma membrane of the IHCs. White arrowheads organelles where the signals for mCLING and the two immunostained proteins colocalized. Scale bar 2 μm. d Graphic representation of Pearson’s correlation coefficients: otoferlin and syntaxin 6 (or syntaxin 16) correlate in the mCLING-labeled organelles at the top and nuclear levels. Vglut3 correlates best with otoferlin at the basal level. At least 100 organelles were analyzed for each condition. Error bars SEMs. e Model of membrane recycling in IHCs. Organelles with a different molecular composition recycle membrane in different regions, taking up mCLING. Apical endocytosis takes up the membrane into round organelles, a sizeable proportion of which is similar to late endosomes (light blue). Endocytosis in the top and nuclear regions reaches tubular organelles containing otoferlin and two endosome markers, syntaxin 16 and syntaxin 6. This suggests that these organelles participate in constitutive pathways, probably by maintaining membrane traffic between the plasma membrane and the trans-Golgi. At the base of the cell, stimulation induces the formation of membrane infoldings and cisterns that are characterized by the presence of Vglut3, Rab3 and also otoferlin. (c–e modified from Revelo et al. 2014, reprinted with permission from © 2014 Revelo et al.)
Mentions: Currently, in IHCs, three distinct mechanisms are considered to mediate endocytosis: slow CME, fast bulk endocytosis and potentially kiss-and-run or ‘ultrafast’ endocytosis (Neef et al. 2014). CME is the main pathway of membrane retrieval for mild stimulation and proceeds at a constant rate; it represents the linear component of endocytosis following exocytosis of the RRP (Fig. 3a). This mechanism is not only inhibited by the clathrin-inhibitor pitstop-2 but also by disruption of dynamin 1 via pharmacological and genetic means (Neef et al. 2014). None of these manipulations seem to affect exocytosis. In contrast, a different study reported inhibition of sustained exocytosis by the presumptive dynamin inhibitor dynasore but did not investigate endocytic membrane retrieval (Duncker et al. 2013). Finally, when exocytosis exceeds three to four RRP equivalents, IHCs additionally recruit a faster mode of membrane retrieval, which proceeds with an exponential time course within a few seconds. It has been proposed to represent bulk endocytosis (Neef et al. 2014; Fig. 3a’) and, indeed, there is plenty of evidence for the invagination and fission of large stretches of plasma membrane in the vicinity of hair cell AZs (Lenzi et al. 2002; Frank et al. 2010; Pangršič et al. 2010; Kamin et al. 2014; Neef et al. 2014; Revelo et al. 2014). Both mechanisms seem to engage in different phases of release: CME supports vesicle cycling during mild stimulation but bulk endocytosis finally occurs after prolonged stimulation, providing a mechanism that assures the balance between exo- and endocytosis in IHCs and thus, assures high release rates (Neef et al. 2014).Fig. 3

Bottom Line: Accumulating evidence indicates a highly specialized molecular composition and structure of the presynapse, adapted to suit these high functional demands.Relating structure and function has become an important avenue in addressing these points and has been applied to normal and genetically manipulated hair cell synapses.Here, we review some of the exciting new insights gained from recent studies of the molecular anatomy and physiology of IHC ribbon synapses.

View Article: PubMed Central - PubMed

Affiliation: Molecular Architecture of Synapses Group, Institute for Auditory Neuroscience and InnerEarLab, University Medical Center Göttingen, Göttingen, Germany, cwichma@gwdg.de.

ABSTRACT
In the mammalian cochlea, sound is encoded at synapses between inner hair cells (IHCs) and type I spiral ganglion neurons (SGNs). Each SGN receives input from a single IHC ribbon-type active zone (AZ) and yet SGNs indefatigably spike up to hundreds of Hz to encode acoustic stimuli with submillisecond precision. Accumulating evidence indicates a highly specialized molecular composition and structure of the presynapse, adapted to suit these high functional demands. However, we are only beginning to understand key features such as stimulus-secretion coupling, exocytosis mechanisms, exo-endocytosis coupling, modes of endocytosis and vesicle reformation, as well as replenishment of the readily releasable pool. Relating structure and function has become an important avenue in addressing these points and has been applied to normal and genetically manipulated hair cell synapses. Here, we review some of the exciting new insights gained from recent studies of the molecular anatomy and physiology of IHC ribbon synapses.

No MeSH data available.


Related in: MedlinePlus