Biology subjects

Axelrod, D.

Publications and source records attributed to Axelrod, D..

4 recordsLinked to original sources

bioRxiv2024

A PACAP-activated network for secretion requires coordination of calcium influx and calcium mobilization

Chromaffin cells of the adrenal medulla transduce sympathetic nerve activity into stress hormone secretion. The two neurotransmitters principally responsible for coupling cell stimulation to secretion are acetylcholine and pituitary adenylate activating polypeptide (PACAP). In contrast to acetylcholine, PACAP evokes a persistent secretory response from chromaffin cells. However, the mechanisms by which PACAP acts are poorly understood. Here, it is shown that PACAP induces sustained increases in cytosolic Ca2+ which are disrupted when Ca2+ influx through L-type channels is blocked or internal Ca2+ stores are depleted. PACAP liberates stored Ca2+ via inositol trisphosphate receptors (IP3Rs) on the endoplasmic reticulum (ER), thereby functionally coupling Ca2+ mobilization to Ca2+ influx and supporting Ca2+-induced Ca2+-release. These Ca2+ influx and mobilization pathways are unified by an absolute dependence on phospholipase C epsilon (PLC{varepsilon}) activity. Thus, the persistent secretory response that is a defining feature of PACAP activity, in situ, is regulated by a signaling network that promotes sustained elevations in intracellular Ca2+ through multiple pathways.

cell biology↗

bioRxiv2021

VAMP2 AND SYNAPTOTAGMINS ARE RELATIVELY IMMOBILE ON CHROMAFFIN GRANULE MEMBRANES: IMPLICATIONS FOR MEMBRANE FUSION AND FUSION PORE EXPANSION

Although many of the proteins of secretory granules have been identified, little is known about their molecular organization and diffusion characteristics. Granule-plasma membrane fusion can only occur when proteins that enable fusion are present at the granule-plasma membrane contact. Thus, the mobility of granule membrane proteins may be an important determinant of fusion pore formation and expansion. To address this issue, we measured the mobility of (fluorophore-tagged) vesicle associated membrane protein 2 (VAMP2), synaptotagmin 1 (Syt1), and synaptotagmin 7 (Syt7) in chromaffin granule membranes in living chromaffin cells. We used a method that is not limited by standard optical resolution. A bright flash of strongly decaying evanescent field ([~]80 nm exponential decay constant) produced by total internal reflection (TIR) was used to photobleach GFP-labeled proteins in the granule membrane. Fluorescence recovery occurs as unbleached protein in the granule membrane distal from the glass interface diffuses into the more bleached proximal regions, thereby enabling the measurement of diffusion coefficients. The studies revealed that VAMP2, Syt1, and Syt7 are relatively immobile in chromaffin granules membranes with diffusion constants of [≤] 3 x 10-10 cm2/s. Utilizing these diffusion parameters and the known density of VAMP2 and Syt 1 on synaptic vesicles, we estimated the time required for these proteins to arrive at a nascent fusion site to be tens of milliseconds. We propose that the mobilities of secretory granule SNARE and Syt proteins, heretofore unappreciated factors, influence the kinetics of exocytosis and protein discharge. Significance StatementIn eukaryotic cells, secretory vesicles fuse with the plasma membrane to secrete chemical transmitters, hormones and proteins that enable diverse physiological functions including neurotransmission. Fusion proteins need to be assembled at the fusion site in sufficient number in order to enable membrane fusion. However, the diffusion characteristics of fusogenic proteins on secretory vesicles remained unknown. Here we used a novel method not limited by standard optical resolution to measure the diffusion of VAMP2 and synaptotagmins on chromaffin granule membranes. We found they have limited mobility. The time required for these proteins to reach the granule-plasma membrane contact site suggests that their limited mobility likely influences the kinetics of membrane fusion and subsequent fusion pore expansion.

biophysics↗

bioRxiv2018

Optimization of Dose Schedules for Chemotherapy of Early Colon Cancer Determined by High Performance Computer Simulations

Cancer chemotherapy dose schedules are conventionally applied intermittently, with dose duration of the order of hours, intervals between doses of days or weeks, and cycles repeated for weeks. The large number of possible combinations of values of duration, interval, and lethality has been an impediment to empirically determine the optimal set of treatment conditions. The purpose of this project was to determine the set of parameters for duration, interval, and lethality that would be most effective for treating early colon cancer. An agent-based computer model that simulated cell proliferation kinetics in normal human colon crypts was calibrated with measurements of human biopsy specimens. Mutant cells were simulated as proliferating and forming an adenoma, or dying if treated with cytotoxic chemotherapy. Using a high performance computer, a total of 28,800 different parameter sets of duration, interval, and lethality were simulated. The effect of each parameter set on the stability of colon crypts, the time to cure a crypt of mutant cells, and the accumulated dose was determined. Of the 28,800 parameter sets, 434 parameter sets were effective in curing the crypts of mutant cells before they could form an adenoma and allowed the crypt normal cell dynamics to recover to pretreatment levels. A group of 14 similar parameter sets produced a minimal time to cure mutant cells. A different group of 9 similar parameter sets produced the least accumulated dose. These parameter sets may be considered as candidate dose schedules to guide clinical trials for early colon cancer.

cancer biology↗

bioRxiv2017

Identification of β-synuclein on secretory granules in chromaffin cells and the effects of α- and β-synuclein on BDNF discharge following fusion

Synuclein is strongly implicated in the pathogenesis of Parkinsons disease as well as in other neurodegenerative diseases. However, its normal function in cells is not understood. The N-termini of -, {beta}-, and {gamma}-synuclein are comprised of seven 11-amino acid repeats that are predicted to form amphipathic helices. -Synuclein binds to negatively charged lipids, especially small vesicles and tubulates and vesiculates lipids. The membrane-binding and membrane-curving abilities raise the possibility that synuclein could alter cellular processes that involve highly curved structures. In the present study we examined the localization of endogenous synuclein in bovine chromaffin cells by immunocytochemistry and its possible function to control protein discharge upon fusion of the granule with the plasma membrane by regulating the fusion pore. We found with quantitative immunocytochemistry that endogenous {beta}-synuclein associates with secretory granules. Endogenous -synuclein only rarely is found on secretory granules. Overexpression of -synuclein but not {beta}-synuclein quickened the median duration of the post-fusion discharge of BDNF-pHluorin by 30%, consistent with -synuclein speeding fusion pore expansion.

neuroscience↗