Ciliogenesis
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Ciliogenesis is defined as the building of the cell's antenna (
primary cilia The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike pr ...
) or extracellular fluid mediation mechanism (
motile cilium The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...
). It includes the assembly and disassembly of the cilia during the
cell cycle The cell cycle, or cell-division cycle, is the sequential series of events that take place in a cell (biology), cell that causes it to divide into two daughter cells. These events include the growth of the cell, duplication of its DNA (DNA re ...
. Cilia are important appendages of cells and are involved in numerous activities such as
cell signaling In biology, cell signaling (cell signalling in British English) is the Biological process, process by which a Cell (biology), cell interacts with itself, other cells, and the environment. Cell signaling is a fundamental property of all Cell (biol ...
, processing developmental signals, and directing the flow of fluids such as mucus over and around cells. Due to the importance of these cell processes, defects in ciliogenesis can lead to numerous human diseases related to non-functioning cilia known as
ciliopathies Ciliopathies are a group of genetically diverse disorders caused by defects in the structure or function of the primary cilium, a highly specialized and evolutionarily conserved organelle found in nearly all eukaryotic cells. The primary cili ...
.


Assembly

Primary cilia are found to be formed when a cell exits the cell cycle. Cilia consist of four main compartments: the
basal body A basal body (synonymous with basal granule, kinetosome, and in older cytological literature with blepharoplast) is a protein structure found at the base of a eukaryotic undulipodium (cilium or flagellum). The basal body was named by Theodor Wi ...
at the base, the transition zone, the axenome which is an arrangement of nine doublet
microtubules Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27  nm and have an inner diameter between 11 an ...
and considered to be the core of the cilium, and the ciliary membrane. Primary cilia contain nine doublet microtubules arranged as a cylinder in their axenome and are denoted as a 9+0 pattern. Motile cilia are denoted as a 9+2 pattern because they contain two extra microtubules in the center of the cylinder that forms the axenome. Due to differences between primary and motile cilia, differences are seen in the formation process. Ciliogenesis occurs through an ordered set of steps. Basal bodies migrate to the surface of the cell and attach to the cell
cortex Cortex or cortical may refer to: Biology * Cortex (anatomy), the outermost layer of an organ ** Cerebral cortex, the outer layer of the vertebrate cerebrum, part of which is the ''forebrain'' *** Motor cortex, the regions of the cerebral cortex i ...
. Along the way, the basal bodies attach to membrane
vesicles Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane * Synaptic vesicle In a neuron, synaptic vesicles (or neurotransmitter vesicles) s ...
that fuse with the
plasma membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...
of the cell. The alignment of cilia is determined by the positioning and orientation of the basal bodies at this step. Once the alignment is determined, axonemal microtubules extend from the basal body and forming the cilia. Proteins must be synthesized in the cytoplasm of the cell and cannot be synthesized within cilia. For the cilium to elongate, proteins must be selectively imported from the cytoplasm into the cilium and transported to the tip of the cilium by
intraflagellar transport Intraflagellar transport (IFT) is a bidirectional motility along axoneme microtubules that is essential for the formation (ciliogenesis) and maintenance of most eukaryotic cilia and flagella. It is thought to be required to build all cilia that as ...
(IFT). Once the cilium is completely formed, it continues to incorporate new
tubulin Tubulin in molecular biology can refer either to the tubulin protein superfamily of globular proteins, or one of the member proteins of that superfamily. α- and β-tubulins polymerize into microtubules, a major component of the eukaryotic cytosk ...
at the tip of the cilia while older tubulin is simultaneously degraded. This requires an active mechanism that maintains ciliary length. Impairments in these mechanisms can affect the motility of the cell and cell signaling between cells. There are two noted types of ciliogenesis: compartmentalized and cytosolic. Most cells undergo compartmentalized ciliogenesis in which cilia are enveloped by extensions of the plasma membrane for the entirety of development. In cytosolic ciliogenesis, the axenome must interact with proteins in the cytoplasm therefore it is directly exposed to the cytoplasm. In some cells, cytosolic ciliogenesis occurs after compartmentalized ciliogenesis.


Disassembly

Cilia disassembly is much less understood than cilia assembly. From recent discoveries, three distinct types of cilia disassembly have been identified. One variety of cilia disassembly occurs when the length of the cilia is gradually reduced until it is no longer functional. Another category of cilia disassembly is shedding where cilia are severed from the main cell body. An example of this is ''Chlamydomonas'' in which a severing enzyme known as katanin separates basal bodies from axenomes. In some organisms, a third method of cilia disassembly has been seen in which the entire axenome is internalized and then later disintegrated. Cilia presence is seen to be inversely related to the progression of the cell cycle which can be seen by assembly occurring during cellular quiescence and disassembly occurring when the cell cycle is stimulated.


Regulation

Different cells use their cilia for different purposes, such as sensory signaling or the movement of fluid. For this reason, when cilia form and how long they are can differ from cell to cell. The processes controlling ciliary formation, degradation, and length must be regulated to ensure that each cell is able to perform its necessary tasks. Each type of cell has an optimal length for its cilia which must be regulated to ensure optimal function of the cell. Some of the same processes that are used to control the formation and removal of cilia (such as IFT) are thought to be used in the regulation of cilia length. Cilia length also differs depending on where a cell is in the cell cycle. Three categories of molecular events that potentially regulate cilia disassembly include activation of AurA kinase and
deacetylation : In chemistry, acetylation is an organic esterification reaction with acetic acid. It introduces an acetyl group into a chemical compound. Such compounds are termed ''acetate esters'' or simply ''acetates''. Deacetylation is the opposite react ...
of microtubules,
depolymerization Depolymerization (or depolymerisation) is the process of converting a polymer into a monomer or a mixture of monomers. This process is driven by an increase in entropy. Ceiling temperature The tendency of polymers to depolymerize is indicated by ...
of microtubules, and ciliary membrane remodeling. Cilia regulation is grossly understudied; however, dysregulation of ciliogenesis is linked to several diseases.


Ciliopathies

Ciliary defects can lead to a broad range of human diseases known as
ciliopathies Ciliopathies are a group of genetically diverse disorders caused by defects in the structure or function of the primary cilium, a highly specialized and evolutionarily conserved organelle found in nearly all eukaryotic cells. The primary cili ...
that are caused by mutations in ciliary proteins. Because of how widespread cilia are, defects can cause ciliopathies in many different regions of the body. Cilia also play a role in cell signaling and the cell cycle therefore defects to them can have a serious impact on the cell’s ability to function. Some common ciliopathies include
primary ciliary dyskinesia Primary ciliary dyskinesia (PCD) is a rare, autosomal recessive genetic ciliopathy, that causes defects in the action of cilia lining the upper and lower respiratory tract, sinuses, Eustachian tube, middle ear, fallopian tube, and flagella of spe ...
,
hydrocephalus Hydrocephalus is a condition in which cerebrospinal fluid (CSF) builds up within the brain, which can cause pressure to increase in the skull. Symptoms may vary according to age. Headaches and double vision are common. Elderly adults with n ...
, polycystic liver and
kidney disease Kidney disease, or renal disease, technically referred to as nephropathy, is damage to or disease of a kidney. Nephritis is an Inflammation, inflammatory kidney disease and has several types according to the location of the inflammation. Infla ...
, some forms of retinal degeneration,
nephronophthisis Nephronophthisis is a genetic disorder of the kidneys which affects children. It is classified as a medullary cystic kidney disease. The disorder is inherited in an autosomal recessive In genetics, dominance is the phenomenon of one variant ...
,
Bardet–Biedl syndrome Bardet–Biedl syndrome (BBS) is a ciliopathic human genetic disorder that produces many effects and affects many body systems. It is characterized by rod/cone dystrophy, polydactyly, central obesity, hypogonadism, and kidney dysfunction in ...
,
Alström syndrome Alström syndrome (AS), also called Alström–Hallgren syndrome, is a very rare autosomal recessive genetic disorder characterised by childhood obesity and multiple organ dysfunction. Symptoms include early-onset type 2 diabetes, cone-rod dyst ...
, and
Meckel–Gruber syndrome Meckel-Gruber syndrome is a rare, lethal ciliopathic genetic disorder, characterized by renal cystic dysplasia, central nervous system malformations (occipital encephalocele), polydactyly (postaxial), hepatic developmental defects, and pulmonary ...
.


References

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