Association of calcium buffering capacity and local inflammatory reactions in injury of motor neurons

Motor neurons that exhibit differences in vulnerability to degeneration have been identified in motor neuron disease and in its animal models. The oculomotor and hypoglossal neurons are regarded as the prototypes of the resistant and susceptible cell types, respectively. Because an increase in the level of intracellular calcium has been proposed as a feature amplifying degenerative processes, we earlier studied the calcium increase in these motor neurons after axotomy in Balb/c mice and demonstrated a correlation between the susceptibility to degeneration and the intracellular calcium increase, with an inverse relation with the calcium buffering capacity, characterized by the parvalbumin or calbindinD28k content. Because the differential susceptibility of the cells might also be attributed to their different cellular environments, in the present experiments, with the aim of verifying directly that a higher calcium buffering capacity is indeed responsible for the enhanced resistance, motor neurons were studied in their original milieu in mice with a genetically increased parvalbumin level. The changes in intracellular calcium level of the hypoglossal and oculomotor neurons after axotomy were studied electron microscopically at a 21-day interval after axotomy, during which time no significant calcium increase was detected in the hypoglossal motor neurons, the response being similar to that of the oculomotor neurons. The hypoglossal motor neurons of the parental mice, used as positive controls, exhibited a transient, significant elevation of calcium. These data provide more direct evidence of the protective role of parvalbumin against the degeneration mediated by a calcium increase in the acute injury of motor neurons. J. Comp. Neurol. 518:1946–1961, 2010. VC 2009 Wiley-Liss, Inc. INDEXING TERMS: calcium; calcium buffering; parvalbumin; axotomy; neuroprotection Motor neuron disease (or amyotrophic lateral sclerosis [ALS]) is a fatal degenerative disorder of the nervous system that affects primarily the upper and lower motor neurons (Rowland and Shneider, 2001). Its etiology is basically unknown, although several mechanisms have been proposed, which probably contribute synergistically to the destruction of the motor neurons (Boillée et al., 2006; Bendotti and Carri, 2009; Rothstein, 2009). The sequence of events during these pathologic processes is mostly known, and some aspects of their reciprocal contribution to the propagation of the injury cascade have been well described (Mattson, 1998). Our research focuses on the role of the increase in the intracellular calcium level of the motor neurons during degeneration. We hypothesize that the inability of the motor neurons to cope successfully with a nonphysiologic calcium load after or during injury may be one of the key events maintaining the complex destructive machinery through a positive feedback of the elementary steps, and an elevated calcium buffering capacity may attenuate the degree of the lesion during stress conditions (Appel et al., 2001; Siklós and Appel, 2005). Support for this hypothesis has emerged from clinical observations and experimental studies. Patients with ALS present an increased Grant sponsor: the Hungarian Scientific Research Fund; Grant number: OTKA-T 048718; Grant sponsor: the Hungarian National Office for Research and Technology; Grant number: GVOP-3.2. 1. 2004-04-0052/3.0; Grant number: TÁMOP-4.2. 2/08/1/2008-0002. *CORRESPONDENCE TO: László Siklós, Ph.D., Laboratory of Molecular Neurobiology, Institute of Biophysics, Biological Research Center, P.O. Box 521, H-6701 Szeged, Hungary. E-mail: siklos@brc.hu VC 2009 Wiley-Liss, Inc. Received July 3, 2009; Revised November 24, 2009; Accepted November 24, 2009 DOI 10.1002/cne.22312 Published online December 17, 2009 in Wiley InterScience (www.interscience. wiley.com) 1946 The Journal of Comparative Neurology | Research in Systems Neuroscience 518:1946–1961 (2010) RESEARCH ARTICLE