Respuesta de las células gliales al daño neuronal

Abstract

La mayoría de los estudios sobre la activación glial utilizan cultivos puros o mixtos de células gliales, y en general, los agentes que se emplean para inducirla ejercen directamente sobre las células gliales. En contraste, hay pocos trabajos donde se considere la muerte neuronal como el estímulo que se desencadena la activación glial. Se desconoce la señal que desencadena la activación glial en respuesta al daño neuronal, pero se sugiere que tanto alteraciones en los contactos neurona-glía como la presencia de determinados factores solubles secretados por las neuronas dañadas pueden jugar un papel importante en este proceso. En este trabajo se profundizó en el conocimiento del papel de la glía en respuesta al daño neuronal y del proceso de activación glial desencadenado por dos modelos de muerte neuronal: la muerte neuronal por excitotoxicidad mediante concentraciones elevadas de glutamato y la muerte neuronal por apoptosis mediante la deprivación de K+ del medio de cultivo. Se ha comprobado que las células gliales de cultivos neurona-glía de cerebelo responden al daño neuronal con cambios funcionales asociados con una activación glial, tal como ocurre in vivo. Sin embargo, aunque en los dos modelos experimentales de daño neuronal utilizados (excitotoxicidad y apoptosis) se produce la muerte de la mayoría de las neuronas a las 24 h, hay diferencias en la respuesta de los distintos parámetros gliales evaluados en función del tipo de muerte neuronal inducida. En respuesta a la muerte neuronal por excitotoxicidad, en las células gliales hay producción o activación de factores relacionados con una respuesta proinflamatoria, incrementan su proliferación y su actividad fagocítica. En respuesta a la muerte neuronal por apoptosis las células gliales no producen moléculas proinflamatorias ni proliferan, pero su actividad fagocítica se induce de manera más rápida que en el modelo de excitotoxicidad. En este caso, la activación glial queda restringida a la detección y eliminación de las neuronas dañadas sin que haya producción de factores que puedan amplificar la activación glial y transformarla en nociva. Los resultados obtenidos muestran que el estado en que se encuentran las células gliales puede tener un papel importante en la respuesta neuronal a estímulos nocivos. Esto podría ser de gran importancia para el diseño de estrategias que permitan potenciar aquellas propiedades protectoras de las células gliales activadas o inhibir aquellas que son nocivas para las neuronas, con la finalidad de favorecer los mecanismos de neuroprotección ante un determinado estímulo nocivo.

<I>In physiological conditions, glial cells play a key role in the normal function of the central nervous system (CNS) both during development and in the adult. Moreover, they are essential to the response of the CNS to pathological conditions. Astroglial and microglial cells respond to neuronal damage with morphological and functional changes and thus become reactive or activated glial cells. Given the complexity of studying glial activation in vivo, numerous authors have approached the subject using in vitro experimental models. However, most experimental approaches use glial cell cultures, and/or the stimulus used to induce glial activation is an in?ammatory agent or a mixture of cytokines that have a direct effect and cause signi?cant modi?cations of glial cells. In contrast, comparatively few studies have addressed glial activation in the presence of neurons employing neuronal damage as the inducing stimulus. The nature of the signal that induces glial activation in response to neuronal damage remains unknown, although direct contact between neuronal and glial cells as well as soluble factors delivered by the damaged neurons probably play an important role.<br/>In the present work, we have studied glial activation occurring in cerebellar neuronal-glial cell cultures in response to different forms of neuronal death. First, these neuronal-glial cultures were exposed to a high concentration of glutamate, which induces excitotoxic neuronal death. Second, we exposed cerebellar neuronal-glial cultures, which need to be cultured in a medium containing a high concentration of K+ (25 mM K+), to serum-free medium containing a normal concentration of K+ (5 mM K+), a well established and widely used model of apoptosis in cerebellar granule neurons.<br/>Our results show that glial cells in neuronal-glial cultures respond to neuronal damage with functional changes associated with glial activation, as occurs in vivo. However, the pattern of response differs depending on the kind of damage induced in the neurons. Although the two experimental models of neuronal damage used (excitotoxicity and apoptosis) resulted in the death of most neuronal cells after 24 h, differences were observed in the response of the various glial parameters evaluated.<br/>In the presence of excitotoxic neuronal death, glial cells increase their production of agents associated with an in?ammatory response, as well as proliferate and become phagocytic. This appears to be a drastic reactive response of glial cells that, on the one hand, clears the extracellular milieu of any neuronal debris or factors released by the dying neurons that may negatively affect the cellular homeostasis, and on the other, results in the production of factors that in turn can be deleterious for the remaining live cells. In contrast, glial cells do not produce pro-in?ammatory molecules in the presence of apoptotic neuronal death, but phagocytic activity is quickly induced. In this case, glial activation appears to constitute a preventative response resulting from a crosstalk between dying neuronal cells and glial cells, aimed at preventing alterations in the extracellular milieu through the release of injuring factors by dying neurons, as well as limiting the glial response as much as possible.<br/>Our results show that the response of neurons to a certain stimulus depends on the presence of glial cells. Our results also show that different aspects of glial activation are independently regulated in response to neuronal damage. It would be of interest to identify the different signal transduction pathways involved in each case. In this way, it may be possible to speci?cally interfere with individual aspects of glial activation in order to favour actions of reactive glial cells that could help neurons to overcome a negative stimulus and, likewise, to inhibit activities that enhance neuronal damage. </i>