Mitochondria-activated cisternae generate the cell specific vesicles in auditory hair cells

Abstract

A dense population of vesicles largely fills the infranuclear compartment of gerbil inner hair cells (IHCs). Although the nature of the cargo in these vesicles has not been determined, the absence of a Golgi apparatus from the IHC's basal compartment suggests that the vesicles lack the glycosylated protein that Golgi cisternae would provide. Instead, they likely possess neurotransmitter and function as synaptic vesicles. The morphologic mechanism for generating the vesicles also remains unexplained. Ultrastructural examination revealed a few discrete clusters of mitochondria in the IHC's basal compartment. The clustered mitochondria made contact either with intermingling single cisternae or with one end of an unique set of polarized parallel cisternae. Both of these cisternal forms belong to a novel, mitochondria-activated category of cisternae which transforms into aligned segments where contacting mitochondria. Mitochondria-activated cisternae also envelope the vesicles in Hensen bodies of outer hair cells (OHCs). Coexistence of the mitochondria-activated cisternae with a specialized population of cytoplasmic vesicles in both IHCs and OHCs implicated this type of cisterna in synthesis of the cell specific vesicles. Assumedly, the mitochondria-activated cisternae possess an ATPase of the Class IV type. This class of enzymes, also designated flippases, translocates aminophospholipid from the outer to inner leaflet of the lipid bilayer and appears thereby to induce a lipid asymmetry which leads to cisternal segmentation and then vesiculation. In support of such an interpretation, RT-PCR analysis demonstrated the presence of Class IV ATPase in the Organ of Corti.

Fig. 1

A: An inner hair cell shows two clusters of mitochondria (arrows) below and lateral to the nucleus. Bar = 2.0 µm. B: Individual linear cisternae located inferior to a Golgi zone (G) spread throughout an infranuclear cluster of mitochondria. The cisternae transform into aligned dense walled segments (arrows) at sites of mitochondrial contact and thus belong to the mitochondria-activated category of cisternae. Bar = 0.3 µm.

Fig. 2

A: A polarized set of parallel cisternae unique to inner hair cells borders the basolateral plasmalemma. Bleb-like spheres (arrow) bind to two short cisternae slightly separated from the inferior end of five longer parallel cisternae. At their upper terminus the parallel cisternae ascend into a cluster of nine mitochondrial profiles. The upward extensions of the cisternae dismantle into continuously aligned segments where contacting the outer and squeezing between the inner mitochondrial surfaces (arrow-heads). The mitochondrial-cisternal complex lies in a bed composed of a widespread population of presumed synaptic vesicles (Spicer et al., 1999) and could mediate biogenesis of the vesicles. Bar = 0.25 µm. B: Another polarized set consists of 6 parallel cisternae. Bar = 0.5 µm. C: Blebs bind at the inferior end of a set of five parallel cisternae. Bar = 0.3 µm.

Fig. 3

Two Hensen bodies in an outer hair cell consist of vesicles densely aggregated into a ring or sphere. Aligned cisternal fragments of variable length delimit the periphery of the Hensen bodies. These and other nearby cisternae (arrows) segment at points of contact with mitochondria. Bar = 0.3 µm.

Fig. 4

RT-PCR analysis of mRNA encoding type IV ATPase in the organ of Corti. A single band of the predicted size was amplified from each primer pair. The PCR product size is 419 bp for primer set 1 and 221 bp for primer set 2, respectively.

Fig. 5

A schematic representation shows polarized parallel cisternae ascending from short separated segments bordered by blebs (arrow) to the basal edge of a cluster of mitochondria. Apparent extensions of the cisternae spread upward and transform into aligned dense walled segments (arrowheads) where contacting mitochondria.