A deep-etching study of the guinea pig tracheal cilium with special reference to the ciliary transitional region

Genetic mutation of Cep76 results in male infertility due to abnormal sperm tail composition

The transition zone is a specialised gate at the base of cilia/flagella, which separates the ciliary compartment from the cytoplasm and strictly regulates protein entry. We identified a potential new regulator of the male germ cell transition zone, CEP76. We demonstrated that CEP76 was involved in the selective entry and incorporation of key proteins required for sperm function and fertility into the ciliary compartment and ultimately the sperm tail. In the mutant, sperm tails were shorter and immotile as a consequence of deficits in essential sperm motility proteins including DNAH2 and AKAP4, which accumulated at the sperm neck in the mutant. Severe annulus, fibrous sheath, and outer dense fibre abnormalities were also detected in sperm lacking CEP76. Finally, we identified that CEP76 dictates annulus positioning and structure. This study suggests CEP76 as a male germ cell transition zone protein and adds further evidence to the hypothesis that the spermatid transition zone and annulus are part of the same functional structure.

Composition, organization and mechanisms of the transition zone, a gate for the cilium

The cilium evolved to provide the ancestral eukaryote with the ability to move and sense its environment. Acquiring these functions required the compartmentalization of a dynein-based motility apparatus and signaling proteins within a discrete subcellular organelle contiguous with the cytosol. Here, we explore the potential molecular mechanisms for how the proximal-most region of the cilium, termed transition zone (TZ), acts as a diffusion barrier for both membrane and soluble proteins and helps to ensure ciliary autonomy and homeostasis. These include a unique complement and spatial organization of proteins that span from the microtubule-based axoneme to the ciliary membrane; a protein picket fence; a specialized lipid microdomain; differential membrane curvature and thickness; and lastly, a size-selective molecular sieve. In addition, the TZ must be permissive for, and functionally integrates with, ciliary trafficking systems (including intraflagellar transport) that cross the barrier and make the ciliary compartment dynamic. The quest to understand the TZ continues and promises to not only illuminate essential aspects of human cell signaling, physiology, and development, but also to unravel how TZ dysfunction contributes to ciliopathies that affect multiple organ systems, including eyes, kidney, and brain.

The primary cilium at the crossroads of mammalian hedgehog signaling

Cilia function as critical sensors of extracellular information, and ciliary dysfunction underlies diverse human disorders including situs inversus, polycystic kidney disease, retinal degeneration, and Bardet-Biedl syndrome. Importantly, mammalian primary cilia have recently been shown to mediate transduction of Hedgehog (Hh) signals, which are involved in a variety of developmental processes. Mutations in several ciliary components disrupt the patterning of the neural tube and limb bud, tissues that rely on precisely coordinated gradients of Hh signal transduction. Numerous components of the Hh pathway, including Patched, Smoothened, and the Gli transcription factors, are present within primary cilia, indicating that key steps of Hh signaling may occur within the cilium. Because dysregulated Hh signaling promotes the development of a variety of human tumors, cilia may also have roles in cancer. Together, these findings have shed light on one mechanism by which primary cilia transduce signals critical for both development and disease.

Pre-natal development of rat nasal epithelia. V. Freeze-fracturing on necklaces of primary and secondary cilia of olfactory and respiratory epithelial cells

Many cilium types have at their proximal base a particulated membrane structure, the so-called ciliary necklace. Necklaces of primary and secondary cilia of olfactory receptor cells and ciliated respiratory cells, and of primary cilia of olfactory supporting cells were studied as a function of embryonic age. Strand numbers in necklaces of primary cilia of these cell types do not differ, but they differ significantly from those of necklaces of secondary cilia. Primary cilia have 2 to 4, but most commonly 3, necklace strands. This is true for necklaces of primary cilia of 8 different nasal cell types: olfactory epithelial basal and glandular cells, vomeronasal receptor and supporting cells, and microvillous respiratory epithelial cells, in addition to the 3 cell types mentioned above. Comparison with other systems suggests that primary cilia resemble flagella of eukaryotic flagellates and spermatozoa of some invertebrates with respect to their number of necklace strands. Average numbers of necklace strands in secondary olfactory cilia increase from 3-4 at the 16th and 17th gestational days to 6-7 in adults. Those in secondary respiratory cilia increase from 2-3 at the 18th and 19th gestational days to 5-6 in adults. Longer cilia have more strands than shorter ones. Necklaces often have free strand endings, also in primary cilia, suggesting that they spiral. Comparing the present data with those in the literature suggests that necklace features occurring during reciliation differ from those of de novo ciliogenesis. Primary and secondary cilia share the following qualities: 1) Membrane regions above necklace strands can differ quite drastically from those below the strands.(ABSTRACT TRUNCATED AT 250 WORDS)

Three-dimensional visualizations of the inner ear hair cell of the guinea pig. A rapid-freeze, deep-etch study of filamentous and membranous organelles

The fine structure of the filamentous and membranous organelles in the stereocilia and in the cuticular plate of sensory hair cells from the guinea pig was examined using a rapid-freeze, deep-etch method. In fixed and unfixed tissue the outer surface of the plasma membrane of the stereocilia had numerous surface protrusions of various sizes and shapes, while the protoplasmic fractured face of the membrane had rather sparse intramembrane particles. Many tiny cross links were present between the adjacent actin filaments and between actin filaments and the plasma membrane of the stereocilia. Numerous fibrils radiating from the hair rootlet were attached to the peripheral actin filaments in the cuticular plate. The radiating fibrils differed from the tiny cross links which interconnected the adjacent, randomly-oriented actin filaments in the cuticular plate. These complex structures consisting of actin filaments in the hair rootlets, radiating fibrils, and peripheral actin filaments may play an important role in regulating stereociliary bending.

Structural similarities between kinocilium of vestibular hair cell and tracheal motile cilium in the guinea pig

A rapid-freeze, deep-etch method was used to visualize the membrane characteristics as well as the three-dimensional ultrastructural features of a kinocilium of the guinea pig vestibular hair cell. A kinocilium of the vestibular hair cell had a typical ciliary necklace, consisting of six to seven parallel intramembrane particle arrays. The basal body of a kinocilium had a plate-structure called a terminal plate, in its interior. In the shaft region of a kinocilium, peripheral microtubules were connected to the central microtubules by periodic tiny linkers, called radial spokes. These structural features of a kinocilium were identical to those of the tracheal motile cilium.

Cytoskeletal organization in the supporting cell of the guinea pig organ of Corti

The rapid-freeze, deep-etch method was used to visualize the three-dimensional organization of cytoskeletons in the supporting cells of the guinea pig organ of Corti. Deep-etched replicas showed that both the head and basal portions of the pillar cell were composed of a filamentous network consisting of several kinds of fibrous elements, into which numerous microtubules and actin filaments were tightly inserted. Myosin S1-decoration showed that the main constituent element in such filamentous networks in the basal portion of the pillar cell was the actin and tiny cross-bridges interconnected the randomly oriented adjacent actin filaments.

Three-dimensional visualization of basal body structures and some cytoskeletal components in the apical zone of tracheal ciliated cells

The ciliated cells of tracheal epithelium were mechanically fragmented to remove the cytoplasmic soluble contents, and the apical zone was examined to clarify the three-dimensional structures of basal body and cytoskeletal filaments using freeze-fracture-etch approaches. The basal body was connected to the apical plasma membrane by definite laminae, formerly called alar sheets. The distal one-half of the basal foot was composed of several smooth-surfaced 12-nm fibrils. Intermediate filament networks extended to the lower half plane of the basal body, and enmeshed the basal body tightly by tiny 5- to 8-nm fibrils. Actin core bundles of microvilli also had tiny crosslinking fibrils. Some actin filaments were seen to run horizontally at the upper half plane of the basal body. Tracheal cilated cells also had circular actin filament bundles just inside the zonula adherens as many other epithelial cells. These cytoskeletal networks which enmeshed both basal bodies and core filaments of microvilli may function as a coordinator of ciliary beating.