SSM2

Introduction

Eukaryotic cells contain hair-like projecting organelles, known as cilia and flagella, which are involved in many sensory and motile functions of the human body, thus any abnormalities in their characteristic 9+2 axonemal structure or their functioning can lead to many different disease processes. In my review I will look at the structure and functions of eukaryotic cilia in the human body and the ways in which mutated or abnormal gene expression can result in their malfunction and cause disease, looking specifically at Primary Ciliary Dyskinesia and the DNAH5 gene.

Structure of cilia

Cilia and flagella have very similar structures. Cilia are about 0.25 micrometers in diameter and 2-20 micrometers long (4) and flagella tend to be longer, 10-100micrometers and fewer per cell than cilia. They are both synthesized by and project from structures known as basal bodies which are a type of centriole located at the cells periphery, which anchors cilia to the cells body and cytoskeleton. They are also microtubule organizing centers which control the direction of the movement of the cilia (1).

Both cilia and Flagella are made up of nine outer fused pairs of microtubule doublets (see figure 1) (3)(8), one of which is complete (A- tubule) and one incomplete (B-tubule), which join together via nexin protein links creating a circular network of microtubule doublets(3) (see figure 1) which surrounds two central single microtubules forming the characteristic "9+2" arrangement known collectively as the axoneme, which itself is surrounded by an extension of the plasma membrane from the cell body (see figure 1) (1)(5).

[pic] (8)(Figure 1 – A schematical drawing of the structure of an axoneme of cilia or flagella, which contains the microtubule doublets joined by nexin, as described above, which are joined to the central microtubule pair by radial spokes which project from the microtubule doublets. It also shows the inner and outer dynein arms attached to the microtubule and the plasma membrane that surrounds the whole axoneme)

Attached to each of the microtubule doublets are dynein arms, one to two "inner", longer, hook shaped arms and two to three shorter "outer" straight arms (see figure 1) which play a major role in motility of cilia and thus the disease process, especially in Primary ciliary dyskinesia. Dynein attaches very strongly to the A-tubule of the microtubule doublet via a protein structure known as the stem, this does not detach during “dynein walking” as described below, this structure subsequently attaches to a globular shaped molecule known as the head which contains structures called stalks (B-links) which enable its weak and transient attachment to the B-tubule of the adjacent microtubule doublets, this formation enables the movement of dynein of one A-tubule along the B-tubule of the adjacent microtubule doublet, as described further down.

Also extending from each outer microtubule doublets of the axoneme are radial spokes, which are polypeptide complexes (see figure 1) containing a "head" portion that faces inwards and attaches to the sheath of the central tubules and may play a role in regulating flagellar motion, although its exact function and method of action are not fully understood (1) (3).

The Regulation of Motility of microtubules

Cilia and flagella are almost structurally identical and are both capable of rhythmical motion but they are different in their beating pattern, flagella movement is propeller like which generates force in the same direction as the flagella axis, where as cilia beat in a backwards and forwards motion, with recovery strokes controlled by the basal body, they produce force that is perpendicular to the cilia’s axis (4).

In both cilia and flagella the nexin and radial spokes maintain the axonemal relationship by holding the doublets in place in a circle while the basal bodies anchor the microtubules limiting the sliding of them lengthwise. This ensures that the cilia and flagella bend, and do not lengthen or separate, enabling them to beat and carry out their function. Upon movement, the dynein arms use the ATPase activity located in the globular heads of their dynein heavy chains, to attach an arm (the stalk) to a B-tubule tubulin molecule via a B-link, which subsequently releases the ADP from the dynein arm, via the hydrolysis of ATP, which causes a conformational change in the dynein molecule detaching it from the B-tubule, the stalk is then free to attach another ATP molecule allowing it to bind to another B-tubule tubulin segment, causing the A-tubules to slide (“walk”) along the adjacent B-tubule in a synchronised fashion, known as dynein “walking” (1) (3) (7) (11).

Role of cilia in human health

Cilia are located in almost every cell in the body including those of the paranasal sinuses, middle ears, posterior nose, ependymal lining of the brain and the fallopian tubes as well as sperm which contain a flagella structure, its “tail” for swimming. They have diverse functions, such as locomotion and sensation, involved in maintaining normal functioning of the body and its processes. Thus deformation of cilia can lead to disease processes associated with specific symptoms depending on the types and location of cilia affected.

[pic]

(Figure 4 – A cartoon of the structure of the 3 different types of cilia in the human body, as described in the table bellow, table 1)

|Type of cilia |Arrangement and function |Example in the human body |
|motile cilia |They have a 9+2 arrangement of outer to central microtubules (see figure 4)|human trachea and fallopian tubes |
| |and are rarely found individually, they are normally in large groups that | |
| |coordinate their movement and constantly beat in a single direction. | |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
| intermediate (Primary cilia) |They have a 9+0 formation of outer microtubules to central microtubules but|embryonic node and rhodopsin in the |
| |also contain some radial spokes and dynein heads (see figure4). |retina of the eye |
|non-motile cilia (sensory) |They have a 9+0 formation of outer microtubules to central microtubules and|Detection of smell |
| |do not contain any radial spokes or dynein arms (see figure 4). | |
| |Their function is a sensory one, they act as antennas sensing signaling | |
| |molecules and changes in the extracellular environment thus they can | |
| |coordinate cellular signaling pathways. | |

(Table 1 – The different types of cilia in the human body, their arrangement and general function including specific examples) (5, 11)

Ciliopathys are described as multisystem diseases since cilia are coded for by many genes and are so widely spread throughout the body with such a diverse array of functions, thus a gene mutation, for example, of a gene coding for a certain aspect of cilia, such as DNAH5 coding for proteins in the outer dynein arms, would cause many cilia, at multiple locations in the body, to have an abnormal structure and thus be dysfunctional in motility causing an array of symptoms in multiple body systems, such as that which occurs with Primary ciliary dyskineasia, as described below. Therefore the location of the cilia and how they are affected means that the type and severity of symptoms vary between different diseases that affect different cilia (5). Examples of ciliopathys include Primary Ciliary Dyskinesia (PCD), Nephronophthisis or Senior-Loken syndrome and Polycystic kidney disease (PKD) as well as many more.

Primary Ciliary Dyskinesia (PCD)

In 1933, Kartagener described a unique syndrome characterized by the triad of situs inversus, chronic sinusitis, and bronchiectasis, which he called Kartagener syndrome, also known as primary ciliary dyskinesia or immotile ciliary syndrome. The syndrome is a rare, inherited heterogeneous, autosomal recessive disorder that occurs in 1:20,000 live births (15). Presentation of symptoms of PCD in adolescence and adult life are the same as for childhood but additionally may include ectopic pregnancy in females and reduced fertility in both females and males.

In normal development, before organs are present, cilia beating from the mammalian node causes the rotational flow of fluid in the body creating a gradient of morphogens producing the left/right asymmetry that determine where organs develop in the body. Situs inversus, which occurs in 50% of people with Kartagener syndrome, is caused by the leftward fluid flow produced by dysfunction of the primary cilia in the mammalian node which disrupts the morphogen gradient altering left/right symmetry of the body in development causing organs to be positioned randomly in the body (5) (2) (3) (11). Situs inversus totalis is when a mirror-image reversal of the body’s visceral organs occurs; it normally has no physiologic consequences. Situs ambiguous is less common and causes organs to predominantly develop either the left or right side of the body. Situs ambiguous often results in cardiovascular abnormalities such as double outlet right ventricle or an interrupted inferior vena cava which can result in death; it also causes problems in the lungs, intestine (e.g. volvulous), liver, central nervous system, genitourinary system and musculoskeletal system(5)(6), some of which are described in table 2.

Bronchiectasis in kartageners syndrome is due to dysfunction of the motility of the cilia in the respiratory tract resulting in mucus, containing pathogens, to pool in the lungs, the pathogens are mostly necrotizing bacterial infections that thrive in this environment and cause local and irreversible dilatation of the bronchi which also become inflamed and thus collapse easily often reducing air flow and oxygen and impairing mucus clearance which further damages the lungs causing more dilatation(2, 3, 5, 6).

Chronic sinusitis is similar to Bronchiectasis and is due to immotile cilia of the upper airways causing impaired mucociliary clearance causing pathogens to collect and grow in the mucus resulting in inflammation of the paranasal sinuses of which there are several including the maxillary, frontal, ethmoid and sphenoid sinuses. The sinusitis normally persists over 12 weeks and causes symptoms such malaise, thick green/yellow discharge from the nasal cavity, nasal congestion and pain and tightness in the face, headaches and blurred vision. (3, 5, 6)

Depending on which cilia are affected and how they are affected will determine what symptoms present, thus a range of different additional symptoms can present when different genes or cilia are mutated in different forms of PCD, some of which are described in table 2.
|System effected |symptoms |How cilia malfunction |
|Respiratory |- respiratory distress syndrome |Cilia dysfunction in the trachea and lungs means they |
| |- neonatal pneumonia with no obvious cause |cannot move mucus and debris (such as pathogens and |
| |- chronic productive cough |irritants) from the respiratory tract |
| |- atypical asthma that’s not responsive to treatment | |
| |- Recurrent or persistent atelectasis | |
|Ear, nose, and paranasal |- chronic persistent rhinorrhea and rhinosinusitis |Cilia malfunction causes difficulties with mucus |
|sinuses | |clearance in the upper airways and thus chronic mucus |
| |- Anosmia |pooling in the nasal cavity and paranasal sinuses. |
| |- Recurrent sinusitis | |
| |- Nasal mucosal congestion or obstruction | |
| |- Mucopurulent nasal discharge | |
| |- Nasal polyps | |
| |- Inflammation of tympanic membranes | |
| |- Perforation of the tympanic membrane with purulent discharge | |
| |- halitosis | |
| |- recurrent Otitis media with effusion (OME) | |
| kidney |Polycystic kidney and liver disease | |
| |Biliary atresia | |
|Gastrointestinal system |- oesophageal atresia | |
| |- severe gastro oesophageal reflux | |
| |- volvulous | |
|Central nervous system |Hydrocephalus |The cilia in the brain ventricles cannot beat and thus |
| | |cerebrospinal fluid cannot circulate. |
|Eye symptoms |Retinal degeneration (e.g. retinitis pigmentosa) | |
|Reproductive system |infertility |The flagellar tail on sperm cannot beat and the sperm |
| | |are immotile causing male infertility. |
| | |The cilia lining the fallopian tubes cannot beat and |
| | |thus the ova or zygote cannot move down the fallopian |
| | |tube to the uterus causing female infertility. |
|Embryo |Situs inversus |A leftward fluid flow is produced by abnormal movement |
| | |of the primary cilia in the mammalian node during |
| | |embryogenesis altering the left/right symmetry of the |
| | |body |

(Table 2 – Body systems that may be affected by PCD and how cilia are affected leading to the production of symptoms - these symptoms may but are not always present in PCD) (2, 3, 5, 11)

Genetics in PCD

The ciliary axoneme contains over 200 proteins coded for by multiple genes at different loci, thus many different gene abnormalities involving any of these structural proteins can occur, potentially leading to the onset of primary ciliary dyskinesia (see figure 7), especially outer dynein arms since they are coded for by many genes at different loci’s (see figure 8) and these gene mutations make up a large proportion of PCD mutations (see figure 7). (15)

[pic] (16) (Figure 7 – Pie Chart showing the different structural defects that occur in PCD and their occurrence)

[pic] (Figure 8 – A Chromosomal Ideogram showing genes that potentially code for outer dynein arm proteins, thus if any of these become mutated they can potentially lead to PCD (15))

There are many genes that have been identified, when mutated, to be involved in the abnormal functioning of cilia in PCD but only a few of which are known, these include DNA H5 (Ciliary dynein heavy chain 5), DNAI1 (Dynein, axonemal, intermediate chain) and DNAH 11. (13)

In research Chlamydomonas reinhardtii, a type of alga, is used to study genes involved in PCD because its 2 flagella are very closely related in structure to cilia in humans, thus studies on the cilia and mutations in genes can be carried out relating the C.reinhardtii gene to its human orthologs( see figure 5) (13).

[pic] (Figure 5 – Schematic showing the structure of the C.reinhardtii outer dynein arm, and the human genes indicating the corresponding proteins they code for in a human outer dynein arm)(15)

In this review I will look closely at DNAH5 gene, located on chromosome 5p15-p14 spanning 250 kb and containing 79 exons as well as 1 alternative first exon, which provides many sites for potential DNA damage resulting abnormal outer dynein arms and thus PCD. Research using Northern blot analyses has shown DNAH5 gene is specifically expressed in the brain, lungs and testis, resulting in symptoms in these locations when the gene is mutated, as described above. (13)

DNAH5 is the human ortholog of C. reinhardtiis dynein [pic]-heavy chain in the outer dynein arm (see figure 5). It codes for a large dynein heavy chain protein consisting of about 4600 amino acid residues with an N-terminal tail, which binds dynein to the A-tubule of the microtubule doublet and thus is the ATP insensitive end, at its other end it attaches to a C-terminal globular head, consisting of a hexamric ring of six AAA domains containing the ATPase activity and branches that bind transiently to the B-tubule of a microtubule doublet and thus are the ATP sensitive end. (15) Thus when DNAH5 is mutated it cannot bind to microtubules effectively or carryout “dynein waling” affecting its motility and functioning.

Most of the mutations in DNAH5 were found to be located in exons 34, 50, 63, 76, and 77 and most were nonsense mutations, although missense, frameshift and splicing mutations were also identified. These mutations were shown to cause slow beating of or absent or shortened outer dynein arms which affects cilia beating and function causing the symptoms, described in table 1 above, such as Bronchiectasis(13).

High resolution immunofluorescence imaging of cells collected via brush sampling of the respiratory tract cells which were then treated with an antibody directed against DNAH5, showed that in people with mutated DNAH5 the subsequent proteins have been shown to be mislocalized and accumulated around the basal body and in the cytoplasm at the cilias base and not spread equally along the entire ciliary axonemes, as they normally are, affecting their ability to carryout dynein “walking” and thus their motility and functioning. (17) (19)

Screening and Diagnosis

There are many way of diagnosis and screening for PCD, most of which measures the ability of cilia to carry out its functions and others measure the symptom progression, all these tests can be carried out by simple out-patient procedures. Although diagnosis can be difficult if the respiratory epithelium have secondary damaged due to infection or inflammation, if this occurs ciliated cells are cultured to asses if infection is present and if so what infection it is so it can be treated (2).

The saccharin test is the most common used test for PCD, it assesses mucociliary function. A microtablet of saccharin is placed on the inferior turbinate in the nasal cavity and a time measurement is taken for how long it takes the patient to taste its sweetness, normally its takes less than 20 minutes and in most people 10, in people who suffer from PCD it takes longer. It is not suitable to be used for diagnosis alone, thus abnormal results need confirmation (2).

Electron microscopy of ciliary ultrastructure is the gold standard and a definitive test for PCD. Analysis of the axonemes of cells collected from a nasal scraping of epithelia is studied using an electron microscope. It may show a lack of outer or inner dynein arms or a combined lack of inner and outer dynein arms, a nexin link, microtubule or radial spoke defect all of which occur in PCD (5, 6).

The nasal nitric oxide (NO) measurement measures the amount of NO produced per minute, which is normally synthesized by the respiratory cells from the enzyme nitric oxide synthase (NOS), L-arginine and oxygen with the cofactor tetrahydrobiopterin. It normally has a high concentration in the upper airways in relation to a lower concentration in the lower airways. In PCD nasal NO production decreases reducing the concentration to a tenth of its normal value, a concentration of less than 100 nL/min, suggest PCD, although the reason for this is unknown (12,13).

Ciliary beat pattern analysis is performed by analysis of cilia in slow motion or frame by frame using high resolution digital high-speed video photography which is able view the precise beat pattern and frequency of cilia in 3 different planes to identify any problems with motility of cilia which would be indicative of PCD (see table 3).
|Cilia beat pattern |Structural abnormality |
|completely immotile cilia with very few slow and stiff, low-amplitude |A combined inner and outer dynein arm defect or only an outer dynein arm defect. |
|movements | |
|stiff planar forward-backward motion with greatly reduced amplitude |Isolated inner dynein arm defect or a radial spoke defect. |
|cilia beat in a large circular gyrating motion about the base of the |transposition defect |
|cilium | |

(Table 3 – The three ciliary beat patterns correlated with structural defects causing PCD) (1)(3)

Ciliary beat frequency and waveform analysis is carried out using direct video cinematography or oscillography which would detect an abnormality in cilia motility which would be indicative of PCD (2, 5, 6).

Genetic sequencing analysis can be carried out looking for specific exon and intron regions that have known gene abnormalities involved in PCD, such as DNAH5 (exons 34, 50, 63, 76, 77) and DNAI1 (exons 13, 16, 17). It is performed by taking blood from the person being screened then a genetic sequence analysis is carried out. The detection of biallelic mutations of genes in PCD confirms a PCD diagnosis; identification of monoallelic mutations suggests the person is a carrier, although negative tests do not exclude a PCD diagnosis. Research, using genetic sequence analysis in families know to have PCD, show 23% have mutations in exons (16) (13).

Immunofluorecent staining of cells acquired by transnasal brushing can identify a characteristic DNAH5 staining pattern detecting outer dynein arm abnormalities which cause abnormal cilia functioning causing PCD.

Chest radiograph shows changes caused by chronic bronchitis, pneumonia, dextrocardia and bronchiectasis which are symptoms of PCD (6). Bronchoscopy may also be used to shows any mucosal inflammation and secretion in the airways caused by the effects of PCD. It can also confirm the reversal of bronchial anatomy in those patients with situs inversus.

Semen analysis can be done in males to analyse sperm for any abnormal motility and thus infertility caused by PCD (5).

Management and Treatment

There is currently no cure for PCD thus treatment focuses on the management and relief of symptoms.

Respiratory management prevents the deterioration of lung functions, due to infection, by clearing the airway of mucus, which may contain pathogens, via the use of physiotherapy and physical exercise. Physiotherapy often includes chest percussion, PEP (positive expiratory pressure) mask and jumping and blowing games for children. Research shows exercise is thought to be a more effective bronchodilator than ß-2 agonist medications in PCD (2).

Respiratory monitoring is carried out to monitor the disease progression which includes regular sputum or cough swab cultures, lung function tests, oxyhemoglobin measurements, chest radiographs, high resolution CT of the lungs (which defines the extent of any peribronchial thickening, Bronchiectasis, peripheral or central mucous plugging and alteration of the parenchymal tissue thus monitoring disease progression.)

Endoscopic sinus surgery or polypectomy can be carried out to aid sinus drainage and nasal breathing. Saline nasal douches are used to treat chronic rhinorrhoea and long-term topical nasal steroids are used to treat rhinosinusitis (2).

Ventilation tubes are often used in otitis medis which helps to drain the fluid. The procedure is carried out by cleaning and drying the ear, and administering topical non-ototoxic anti-pseudomonal antibiotic eardrops (e.g. ciprofloxacin) and inserting the tube into the tympanic membrane.

Lobectomy can be carried out in rare cases if the patient has persistent localized bronchiectasis that is progressive even after other less invasive medical treatments. It is also used in cases of recurrent infection in localized non-functioning tissue. Children suffering from PCD should have hearing tests preformed regularly until adolescence and hearing aids should be used if necessary until spontaneous resolution of hearing loss, which is a common occurrence. Speech therapy should be provided for those who have a delayed onset of speech.

Infertility in males is often overcome by fertility treatment, such as intracytoplasmic sperm injection or artificial insemination by donor sperm (AIDS) for male infertility (2). Female infertility can be overcome by fertility treatments such as in vitro fertilization or embryo transfer.

Vaccines are often given as a prophylactic for infections such as pertussis, type b and pneumococcus, measles, Haemophilus influenza which commonly occur when the mucous pools. If an infection is acquired antibiotic therapy is given.

Any limitations in current work and studies

It is often very hard to obtain cells, to study, from those who suffer from PCD that have not undergone secondary damage due to infections or other disease processes, thus obtaining accurate results is difficult.

Further work

Cilia are very complex in their structure, function and pathology due to the fact they are coded for by multiple genes and located on nearly all cells in the body with a wide range of functions. To further understand diseases caused by cilia more work needs to be carried out on the specific differences between the different types of cilia and how their malfunction results in the different disease process, as well as mapping all the genes that control the cilia’s structure and functioning, new models like those used with the Chlamydomonas reinhardtii, as described above may be helpful, such as then new dog pedigree model showing autosomal dominant segregation which is the same as that of PCD. This would then enable further studies to be done on gene replacement therapy to target and replace the mutated genes and thus provide a cure. Also more research needs to be carried out on effective management of PCD symptoms to reduce any secondary effects of the disease process.

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19 http://www.eurolupa.org/project/56.html. Unravelling common human diseases using dog genetics. Last Updated Sunday, 20 April 2008 07:30