Cell Physiology

Chemical messengers and hormones are stored in cells in membrane bound vesicles. The contents of vesicles is released into the extracellular medium upon the fusion of the vesicle membrane with the plasma membrane, a process termed exocytosis. In many cells a stimulus is required to trigger exocytosis (regulated exocytosis), while in others exocytosis proceeds in a continuous fashion (constitutive exocytosis). Practically all cells in human body perform a form of exocytosis. In some cells, such as neurons and endocrine cells, this process is particularly specialized. However, it is also present in adipocytes, cardiomiocytes, immune cells, photoreceptors, glial cells, plant cells and other cell types. Although exocytosis is an ubiquitous process of all eukaryotic cells, the molecular mechanisms regulating it are still unclear. In the last decade many proteins have been discovered to play a role in exocytosis, but the exact order and nature of their interactions underlying regulated exocytosis is unclear. On one side it has been hypothesized that a common molecular mechanism is essential for regulated exocytosis, but the interaction of other molecules with the essential set of molecules contribute to specific functional requirements. On the other side it has been proposed that regulated exocytosis is explained by a sequential molecular model, which states that all vesicles in a cell must undergo a sequence of events in order to fuse with plasma membrane. To test these hypotheses, we study secretory activity in a number of model cell types, especially in pituitary cells from the anterior and intermediate lobes, which secrete prolactin, beta-endorphin and alpha-melanocyte stimulating hormone. These hormones are involved in the bodily response to stress, regulate the immune system, body temperature, body weight and other functions. In adition to this we also study the physiology of mitochondria, secretory activity of single astrocytes, adipocytes, skeletal muscle fibres and other cells. Membrane fusion is an important process also in cell maturation, such as in the formation of skeletal muscle fibres. It also represents a key step in the production of hybrid cells used in the production of monoclonal antibodies and for the preparation of hybrid cells in immunotherapy of cancer. In these applications membranes of two adjacent cells fuse to form one cell-hybrid. In case of immunotherapy the fusion of dendritic cells with tumor cells enables the hybrid to retain properties of antigen presenting cells, but is presenting tumour antigens to other immune cells at the same time. The aim of the "Cell Physiology" programme is therefore on one side to understand the fundamental properties of membrane fusion under physiological and pathological conditions, and on the other side to utilize this knowledge in developing hybrid cells and cell therapy related products.

The conducted research is significant at severallevels. It provides new insights into fundamental cell processes including exocytosis, vesicle transport, cytosolic homeostasis of second messengers and metabolites. Moreover this research provides a model system to study functional aspects of »genomics and proteomics«. In complex biological systems in which several genes and proteins participate, it is very difficult, if not impossible, to monitor physiological processes in living organisms in real-time. Therefore, to study such processes isolated cells and tissues are suitable models, together with dedicated combinations of techniques. The focus in such efforts are properties of functional modules, which shape the function of a single cell and the whole organism. With this aim we are studying the process of regulated exocytosis in pituitary lactotrophs. Dynamics of vesicles before and after the exocytotic merger of vesicles with the plasma membrane is studied also in other cell types, such as astrocytes. Recent experimental evidences revealed that astrocytes are playing a key role in signal processing in the central nervous system, in normal and in pathological conditions. By using most advanced electrophysiological an optophysiological methods the results showed that a single vesicle in Astrocytes contains less than 25 synaptobrevin 2 molecules and that these molecules are clustered. In comparison to neurons where there are over 70 molecules per single vesicle, the paucity of synaptobrevin molecules may explain the relative slowness of regulated exocytosis in astrocytes. Finally, it seems reasonable to hope that this work, while not targeted to any particular disease, will be useful for helping to preserve and promote human health. Anterior pituitary hormones control important bodily functions including growth, development, reproduction, and responses to stress. The release of hormones is controlled by factors released from innervating neurons (pars intermedia), or by circulating hypothalamic factors which via their interaction with specific surface membrane receptors and subsequent activation of intracellular signalling mechanisms control exocytosis. Thus a key to understanding neuroendocrine integration is to understand the mechanisms that control exocytosis. Astrocytes play important roles in the CNS function that have only recently been described, much of this has to be thanked to single cell studies in particular. Addressing the role of regulated exocytosis in cells playing a role in metabolic syndrome and diabetes, will potentially generate new vistas that will help formulate new paradigms for diagnostic/therapeutic approaches. Therefore, the conducted research does not only examine fundamental physiological/biophysical aspects of exocytosis, vesicle traffic, cytosolic signaling with second messengers and metabolites, but additionally reveals new aspects of physiological regulation of hormone and neurotransmitter release, vesicle traffic and cytosolic homeostasis in terms of conditions related to neurodegeneration, brain trauma, metabolic syndrome and diabetes.

Cell Physiology represents an essential discipline for the development of new therapeutic and diagnostic methods in modern molecular medicine. Therefore, the strategy to support fundamental research and applied research at cellular level represents a strategy to deliver and support a much more rapid development of the whole society, including that in Slovenia. Fundamental research is essential for the education of future experts and also lay public. The latter needs to be educated in term of understanding the new developments in cell biology and molecular medicine. For example: the promissing new discoveries in stem cell research, subcellular physiology, cell engineering, cell transplantation, the biology and mechanisms of diseases, the mechanisms of treatment, and others need to be presented to the public. Furthermore, problem solving of universal problems at the highest possible methodological level is increasing the visibility, credibility and competence of the whole society. Expertise in the field of Cell Physiology, which has a tradition of several decades in Slovenia, promisses to support more rapid development for Slovenia, contributing significance for global efforts with the family of other nations.