Biomedical subjects

Joseph C Chen

Publications and source records attributed to Joseph C Chen.

6 recordsLinked to original sources

PubMed2006

Cytokinesis signals truncation of the PodJ polarity factor by a cell cycle-regulated protease.

We demonstrate that successive cleavage events involving regulated intramembrane proteolysis (Rip) occur as a function of time during the Caulobacter cell cycle. The proteolytic substrate PodJ(L) is a polar factor that recruits proteins required for polar organelle biogenesis to the correct cell pole at a defined time in the cell cycle. We have identified a periplasmic protease (PerP) that initiates the proteolytic sequence by truncating PodJ(L) to a form with altered activity (PodJ(S)). Expression of perP is regulated by a signal transduction system that activates cell type-specific transcription programs and conversion of PodJ(L) to PodJ(S) in response to the completion of cytokinesis. PodJ(S), sequestered to the progeny swarmer cell, is subsequently released from the polar membrane by the membrane metalloprotease MmpA for degradation during the swarmer-to-stalked cell transition. This sequence of proteolytic events contributes to the asymmetric localization of PodJ isoforms to the appropriate cell pole. Thus, temporal activation of the PerP protease and spatial restriction of the polar PodJ(L) substrate cooperatively control the cell cycle-dependent onset of Rip.

Bacterial Proteins↗

PubMed2005

Intradural cranial congenital dermal sinuses: diagnosis and management.

OBJECTIVE: The congenital dermal sinus (CDS) is a benign tumor-like entity that has unique anatomical and clinical features. We retrospectively examined our data to determine factors associated with adverse outcomes for cranial-based dermal sinuses. METHODS: We retrospectively examined our data obtained from patients presenting between 1975 and 2002. Sixty-seven patients with a CDS were found. Of these, 20 cases of a cranial CDS were identified, 15 of which had evidence of an intradural component. RESULTS: Surgical treatment of congenital dermal sinuses was accompanied with, in general, excellent results. Poor outcomes were most strongly associated with delays in diagnosis. The most common complications were infection at the surgical site (five patients) and hydrocephalus necessitating ventriculoperitoneal shunt placement (two patients). Permanent neurological deficits were rare. CONCLUSION: Cranial CDSs are unusual lesions found in children. A majority of these lesions may penetrate the dura and lead to significant complications if not promptly identified and treated in an appropriate and timely fashion.

Child↗

PubMed2005

A membrane metalloprotease participates in the sequential degradation of a Caulobacter polarity determinant.

Caulobacter crescentus assembles many of its cellular machines at distinct times and locations during the cell cycle. PodJ provides the spatial cues for the biogenesis of several polar organelles, including the pili, adhesive holdfast and chemotactic apparatus, by recruiting structural and regulatory proteins, such as CpaE and PleC, to a specific cell pole. PodJ is a protein with a single transmembrane domain that exists in two forms, full-length (PodJL) and truncated (PodJS), each appearing during a specific time period of the cell cycle to control different aspects of polar organelle development. PodJL is synthesized in the early predivisional cell and is later proteolytically converted to PodJS. During the swarmer-to-stalked transition, PodJS must be degraded to preserve asymmetry in the next cell cycle. We found that MmpA facilitates the degradation of PodJS. MmpA belongs to the site-2 protease (S2P) family of membrane-embedded zinc metalloproteases, which includes SpoIVFB and YluC of Bacillus subtilis and YaeL of Escherichia coli. MmpA appears to cleave within or near the transmembrane segment of PodJS, releasing it into the cytoplasm for complete proteolysis. While PodJS has a specific temporal and spatial address, MmpA is present throughout the cell cycle; furthermore, periplasmic fusion to mRFP1 suggested that MmpA is uniformly distributed around the cell. We also determined that mmpA and yaeL can complement each other in C. crescentus and E. coli, indicating functional conservation. Thus, the sequential degradation of PodJ appears to involve regulated intramembrane proteolysis (Rip) by MmpA.

Bacterial Proteins↗

PubMed2005

Distinct constrictive processes, separated in time and space, divide caulobacter inner and outer membranes.

Cryoelectron microscope tomography (cryoEM) and a fluorescence loss in photobleaching (FLIP) assay were used to characterize progression of the terminal stages of Caulobacter crescentus cell division. Tomographic cryoEM images of the cell division site show separate constrictive processes closing first the inner membrane (IM) and then the outer membrane (OM) in a manner distinctly different from that of septum-forming bacteria. FLIP experiments had previously shown cytoplasmic compartmentalization (when cytoplasmic proteins can no longer diffuse between the two nascent progeny cell compartments) occurring 18 min before daughter cell separation in a 135-min cell cycle so the two constrictive processes are separated in both time and space. In the very latest stages of both IM and OM constriction, short membrane tether structures are observed. The smallest observed pre-fission tethers were 60 nm in diameter for both the inner and outer membranes. Here, we also used FLIP experiments to show that both membrane-bound and periplasmic fluorescent proteins diffuse freely through the FtsZ ring during most of the constriction procession.

Bacterial Outer Membrane Proteins↗

PubMed2004

Geometrical accuracy of the Novalis stereotactic radiosurgery system for trigeminal neuralgia.

OBJECT: Stringent geometrical accuracy and precision are required in the stereotactic radiosurgical treatment of patients. Accurate targeting is especially important when treating a patient in a single fraction of a very high radiation dose (90 Gy) to a small target such as that used in the treatment of trigeminal neuralgia (3 to 4-mm diameter). The purpose of this study was to determine the inaccuracies in each step of the procedure including imaging, fusion, treatment planning, and finally the treatment. The authors implemented a detailed quality-assurance program. METHODS: Overall geometrical accuracy of the Novalis stereotactic system was evaluated using a Radionics Geometric Phantom Chamber. The phantom has several magnetic resonance (MR) and computerized tomography (CT) imaging-friendly objects of various shapes and sizes. Axial 1-mm-thick MR and CT images of the phantom were acquired using a T1-weighted three-dimensional spoiled gradient recalled pulse sequence and the CT scanning protocols used clinically in patients. The absolute errors due to MR image distortion, CT scan resolution, and the image fusion inaccuracies were measured knowing the exact physical dimensions of the objects in the phantom. The isocentric accuracy of the Novalis gantry and the patient support system was measured using the Winston-Lutz test. Because inaccuracies are cumulative, to calculate the system's overall spatial accuracy, the root mean square (RMS) of all the errors was calculated. To validate the accuracy of the technique, a 1.5-mm-diameter spherical marker taped on top of a radiochromic film was fixed parallel to the x-z plane of the stereotactic coordinate system inside the phantom. The marker was defined as a target on the CT images, and seven noncoplanar circular arcs were used to treat the target on the film. The calculated system RMS value was then correlated with the position of the target and the highest density on the radiochromic film. The mean spatial errors due to image fusion and MR imaging were 0.41+/-0.3 and 0.22+/-0.1 mm, respectively. Gantry and couch isocentricities were 0.3+/-0.1 and 0.6+/-0.15 mm, respectively. The system overall RMS values were 0.9 and 0.6 mm with and without the couch errors included, respectively (isocenter variations due to couch rotation are microadjusted between couch positions). The positional verification of the marker was within 0.7+/-0.1 mm of the highest optical density on the radiochromic film, correlating well with the system's overall RMS value. The overall mean system deviation was 0.32+/-0.42 mm. CONCLUSIONS: The highest spatial errors were caused by image fusion and gantry rotation. A comprehensive quality-assurance program was developed for the authors' stereotactic radiosurgery program that includes medical imaging, linear accelerator mechanical isocentricity, and treatment delivery. For a successful treatment of trigeminal neuralgia with a 4-mm cone, the overall RMS value of equal to or less than 1 mm must be guaranteed.

Equipment Design↗

PubMed2002

Analysis of ftsQ mutant alleles in Escherichia coli: complementation, septal localization, and recruitment of downstream cell division proteins.

FtsQ, a 276-amino-acid, bitopic membrane protein, is one of the nine proteins known to be essential for cell division in gram-negative bacterium Escherichia coli. To define residues in FtsQ critical for function, we performed random mutagenesis on the ftsQ gene and identified four alleles (ftsQ2, ftsQ6, ftsQ15, and ftsQ65) that fail to complement the ftsQ1(Ts) mutation at the restrictive temperature. Two of the mutant proteins, FtsQ6 and FtsQ15, are functional at lower temperatures but are unable to localize to the division site unless wild-type FtsQ is depleted, suggesting that they compete poorly with the wild-type protein for septal targeting. The other two mutants, FtsQ2 and FtsQ65, are nonfunctional at all temperatures tested and have dominant-negative effects when expressed in an ftsQ1(Ts) strain at the permissive temperature. FtsQ2 and FtsQ65 localize to the division site in the presence or absence of endogenous FtsQ, but they cannot recruit downstream cell division proteins, such as FtsL, to the septum. These results suggest that FtsQ2 and FtsQ65 compete efficiently for septal targeting but fail to promote the further assembly of the cell division machinery. Thus, we have separated the localization ability of FtsQ from its other functions, including recruitment of downstream cell division proteins, and are beginning to define regions of the protein responsible for these distinct capabilities.

Alleles↗