Cilium | EPFL Graph Search

The cilium (: cilia; ), is a membrane-bound organelle found on most types of eukaryotic cell. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projection that extends from the surface of the much larger cell body. Eukaryotic flagella found on sperm cells and many protozoans have a similar structure to motile cilia that enables swimming through liquids; they are longer than cilia and have a different undulating motion. There are two major classes of cilia: motile and non-motile cilia, each with a subtype, giving four types in all. A cell will typically have one primary cilium or many motile cilia. The structure of the cilium core called the axoneme determines the cilium class. Most motile cilia have a central pair of single microtubules surrounded by nine pairs of double microtubules called a 9+2 axoneme. Most non-motile cilia have a 9+0 axoneme that lacks the central pair of microtubules. Also lacking are the associated

A mechanical approach to study the bending of the primary cilium in response to fluid flow

Florian Herzog

The primary cilium, a hair-like projection from the cellular membrane, is involved in fluid flow sensing. Bending the primary cilium is known to trigger several signaling pathways. In this study, the primary cilium is modeled as a thin beam undergoing large deflection due to fluid drag forces. For the first time, the bending response is analyzed with a model combining large angle rotations with the assumption of a linear drag force along the ciliary length. In addition, the initial curvature and the angle between the cell membrane and the cilium are integrated into the model. The model is applied on three dimensional confocal images of fluorescent cilia in living cells that are exposed to laminar shear flow. The coordinates of the cilia are computed by a simple and effective image processing algorithm. By fitting the equations of the model to the coordinates of the bent cilia, the flexural rigidity and the angle between the cilium and the cell membrane are estimated. The flexural rigidity of the primary cilium is approximately

{10}^{-23}

Nm, but the values vary for different cilia, which suggests that the mechanical properties of the primary cilia are heterogeneous. Interestingly, the base of the cilium doesn't deflect under fluid flow which means that the primary cilium is firmly anchored. Using immunocytochemistry, the connections of the microtubule network to the base of the cilium could be resolved. The microtubules may provide the mechanical stability of the cilium. Incorporating a linear drag force and allowing a basal tilt is an important step towards a more realistic mechanical model of the primary cilium and towards more accurate values of its flexural rigidity. The model together with the imaging technique is a useful tool to study the mechanical behavior of the primary cilium and will eventually lead to new insight in the poorly understood mechanisms of mechanotransduction.

2010

Gamma-Secretase Dependent Gene Expression

Alexandra Marie-Isabelle Karoline Brigitte Magold

The causes of sporadic Alzheimer's Disease (AD), the most common form of dementia at old age, are still unknown. Familial early onset AD (FAD) is mainly caused by mutations of Presenilins (PS), the active subunits of γ-secretase. Processing of its substrate proteins by γ-secretase generates shorter fragments. In the case of the Amyloid Precursor Protein (APP) these are Aβ species, of which the ratio of Aβ40, which is 40 amino acids (aa) long and Aβ42, 42 aa in length respectively, has been connected with their aggregation and AD plaque formation. Another cleavage product of many γ-secretase substrate proteins are intracellular domains (ICDs), which enter the nucleus and alter gene transcription. Be it via statins, the levels of cAMP, the generation of which is also catalyzed by cilia specific Adenylyl-cyclase III (ACIII) or Wnt pathways, cilia can be connected with various AD relevant proteins, pathways and functions, but their physiological and thus pathological cerebral roles are not understood. We hypothesized that γ-secretase activity alterations impact transcription levels of specific genes and potentially signaling and functional pathways as well as cilia genes in ways relevant to AD. We determined the transcriptional impact of enhanced γ-secretase activity in Chinese hamster ovary (CHO) cells with a cDNA microarray experiment. Gene ontology cluster analysis of the 1981 genes reported by our microarray gave insights into functional groups represented. Matching of known physical protein-protein interactions as reported by the string 8.0 database for 21 genes, which were confirmed to be transcriptionally altered with γ-secretase activity, registered various members of three different Wnt pathways. Consistent with an hypothesized role of cilia, the array also reported two dozen of cilia-related genes. The most decreased transcript levels were of Anti-mitotic exit network homolog 1 (AMN1), a gene of predicted expression in the cilia of sensory olfactory neurons and for which protein data was available in yeast only. Our immunohistochemistry results confirm Amn1 protein expression in mammalian cells, specifically in murine and human brains. We showed that Amn1 protein is localized in cortical neurons and in sharply outlined intracellular structures close to motile cilia and the opposing cell pole of monocilia. Aged APPPS1 transgenic mice, which over express human APP and PS1 with FAD mutations, did neither show differences in lateral ventricular cilia range, cilia density, the percentage of ciliated hippocampal neurons nor ACIII protein levels. Consistently with previous reports of increased cAMP levels in the cerebrospinal fluid of AD patients, we detected significantly increased levels of ACIII protein in human AD cortex samples (AD n=11, non demented n=12). These results support a new focus on the role of cilia-related proteins in AD. They also suggest some limitations of the APPPS1 transgenic animal model. In recent years the impact and role of cilia-related proteins in various neurodegenerative diseases was demonstrated. This PhD thesis provides evidence that AD should be included in the list of neurodegenerative diseases which are being investigated for the impact and role of cilia-related proteins in their pathologies.

EPFL2011