Glial cells and neurons are in intimate communication with each other during central nervous system development and normal brain function. Glial cells monitor and respond to neural activity by conditioning the extacellular milieu, signaling within glial cell networks as well as by sending signals back to neurons. While glial cells lack electrically excitable properties similar to neurons, they do, however, possess a form of Ca2+ based excitability, whereby intracellular Ca2+ signals, propagated as waves serve as long distance signals in response to monitored synaptic activity.
We aim to understand the principles and mechanisms that govern intracellular calcium signals in glial cells and neurons. This focus includes studies on the signal transduction mechanisms of various receptor systems in glial cells as well as the cell biology of calcium signaling in both glia and neurons. One objective is to understand processes that support temporal and spatial characteristics of Ca2+ signals within cells and between cells. A second objective is to understand the precise nature of glial cell signals in response to neuronal activity and the consequence of such signals to CNS function. Finally, this Section aims to describe in detail the ionic basis for cellular excitability in pineal cells.In previous work, we developed a detailed description of the mechanism of Ca2+ wave propagation within glial cell processes. We found that wave propagation was saltatory due to the underlying regenerative Ca2+ release process. In this model, regenerative Ca2+ release is supported by local cellular specializations in the Ca2+ release machinery which we designate as wave amplification sites. These sites are characterized by high density patches of endoplasmic reticulum (ER) proteins such as the inositol 1,4,5-trisphosphate receptors (IP3Rs), sarco-endoplasmic reticulum calcium pumps, calreticulin and at least one mitochondrion in close association. This specialization allows for the enhanced Ca2+ release at these wave amplification sites. In addition to supporting long distance wave propagation, these specialized sites also provide for locally discrete Ca2+ signals. Current research is focussed on characterization of the local Ca2+ release sites with very high spatial and temporal resolution using confocal microscopy.
The aim i s to measure the kinetics of elementary Ca2+ release events. These are the smallest units of local Ca2+ release, presumed to be clusters of IP3R ion channels on the ER, and are known as Ca2+ sparks and Ca2+ puffs. Such characterization of the kinetics will allow us to test the relative contribution of several different independent cellular processes that regulate the gating properties of the ion channel. We are particularly focussed on the role of mitochondria in the regulation of IP3Rs, since mitochondria are always found in close proximity of these ion channels. Preliminary experiments show that mitochondria may regulate the concentration of Ca2+ in the mouth of IP3Rs and thus regulate their gating properties. Changes in mitochondrial membrane potential and intra mitochondrial Ca2+ concentration associated with cytosolic Ca2+ signals are also being investigated. In addition to these functional parameters, we are also studying the morphological correlates of the Ca2+ release function. The distribution of different IP3R isoforms in CNS cells (Glia and neurons) is being studied in both adult and developing rat brains. Fluorescence based immunocytochemistry and high resolution confocal microscopy are being used in conjunction for this purpose. In collaboration with Dr. Carmen Mannella at the high voltage electron microscopy facility in Wadsworth Medical Center, Albany, NY, we are investigating the relationship between ER membran""""""""es and mitochondria in oligodendroglial processes. These studies follow our observation that mitochondria are not only found in close proximity to IP3R clusters in cellular processes, but changes in both mitochondrial membrane potential and matrix Ca2+ concentration occur during cytosolic Ca2+ signals. Our long term goal is to describe in detail the nature of communication between neuronal networks and glial cell networks. We are developing experimental models to investigate the physiological consequences of glial cell signals in response to neuronal activity. For this purpose, methods are being currently developed to image glial cell processes in brain slice preparations.
The aim i s to monitor cell shape and volume changes associated with neuronal activity. Impaired glial cell signaling has been implicated in a number of pathological states in the CNS such as excitotoxicity, brain edema and certain degenerative diseases. It is hoped that a detailed understanding of the glial cell signaling modes will be useful in understanding the pathophysiology of such conditions.
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