Cilia

Question 1

1. Identify the components of cilia.

Answer 1

• Centriole/basal body = core anchors from which cilia are formed; MT rich cylinder with 9 triplet MT’s (A-B-C).
• Axoneme = structural skeleton; formed from doublet MTs (A-B tubules); - end at the basal body, + end at the ciliary tip.
• Help with scaffolding and provide tracks for movement within cilia.
• Transition Zone = links basal body to axoneme and axoneme to ciliary membrane.
• Also a gatekeeper because it limits diffusion of proteins in and out of cilium.
• Makes sure that ciliary membrane is a distinct compartment that can be used for signaling.
• Ciliary Membrane = continuous with cell’s plasma membrane, but is compartmentalized (via transition zone) so that it is distinct with it’s own unique phospholipids and receptor molecules.
• Intraflagellar transport = axonemes are highways.
• IFT proves the movement.
• Transport is bidirectional.
• Anterograde motion heads towards ciliary tip using IFT-B protein complex and kinesin-2 motor family.
• Retrograde motion occurs back towards basal body using IFT-A protein complex and dynein-2 motor family.

Question 2

2. Explain how cilia are assembled.

Answer 2

2 phases:
Centriole phase, and ciliogenesis phase.

1) Basal bodies are from centrioles (the mother centriole).
- Centrioles duplicated in G1-S transition phase along with DNA and is tightly regulated.
- One new and one old centriole goes to each cell.
- Mother basal body is formed by the older centriole.

2) Ciliogenesis occurs in G0 or G1 phase; assembled from the mother centriole.
- Distal end of BB is capped by ciliary vesicle and MT doublets assemble into ciliary vesicle before structure fuses with cell plasma membrane.
- Can have more than 1 cilium per cell – some terminally differentiated cells nucleate >100 cilia/cell.
- Mechanisms are in place that allow formation of many basal bodies, and assembly is uncoupled from cell cycle.

Question 3

3. Explain the differences between motile and sensory cilia.

Answer 3

-Motile and sensory cilia are the 2 classes; motile cilia can have sensory functions.

• Motile cilia = necessary for movement of fluid (respiratory, neural, reproductive).
• Movement produced by axonemal dynein-dependent sliding motion between doublet MTs.
• Motile cilia generally have 9+2 arrangement (can have 9+0 – like nodal cilia).

Sensory/primary cilia = 9+0 MT arrangement and don’t have axonemal dynein arms; generally perform signaling functions.

Question 4

4. Identify signaling pathways that function through cilia and explain why cilia are used for signaling.

Answer 4

Cilia = antennae of the cell.

• Helps concentrate signal with a high receptor surface-to-volume ratio.
• Signal is localized and polarized in a discrete domain of the cell.
• Receptors are positioned away from interfering cellular domains; cilia can function as mechanical detector of flow.
• Cilia can sense physical stimuli (mechanical strain, temperature, osmolality, gravity), light, and chemical stimuli (hormones, GFs, morphogens).
• Hh signaling pathway uses cilia.
• Unstimulated receptor = no Hh, so no Gli transactivation.
• Stimulated receptor = ligand Hh binds PTCH –> active Gli transactivation.
• Other signaling pathways = Wnt, PDGF, FGF, etc.

Question 5

5. Provide examples of how cilia function in development and tissue homeostasis.

Answer 5

• In example of Hh signaling, downstream targets –> limb formation, bone formation and homeostasis, neurogenesis.
• Other examples: tissue/cellular polarity, tissue patterning, left-right axis determination (laterality), neural tube formation, organogenesis, etc.

Example: motile and sensory cilia necessary to establish left-right asymmetry.

• Ciliary node = 9+0 organization that also beat in rotary fashion to produce leftward flow of signals/morphogens.
• Signals detected by sensory cilia –> organogenesis.

Question 6

6. Recognize the clinical features and cilia defects associated with ciliopathies.

Answer 6

Ciliopathies:

• cystic kidneys,
• nephronophthisis,
• obesity,
• polydactyly,
• retinal degeneration,
• cancer, diabetes,
• infertility.

Ciliopathies = rare, pleiotropic, overlapping, structural, diverse, genetically complex (Cep290 = mutations in this gene can produce 4 different clinical outcomes. Suggests 2nd site modifier mutations).

Example:
Bardet-Biedl Syndrome (BBS)
= auto recessive, anosmia (smell is gone), BBS proteins aid in vesicle transport within cilium.
Symptoms = photoreceptor degeneration, mental retardation, neural tube defects, obesity, diabetes, etc.

PKD =
-autosomal dominant and recessive forms (ADPKD and ARPKD)
-ADPKD = mutation in polycystin-1 and polycystin-2.
These genes encode channel proteins involved in Ca signaling.
-Channels sense mechanical flow of urine in kidney to transduce Ca signaling.
Symptoms = renal cysts, liver/pancreas cysts, intracranial aneurysms.