BaxInactivation in Lurcher Mutants Rescues Cerebellar Granule Cells But Not Purkinje Cells or Inferior Olivary Neurons

Abstract

Lurcher is a gain-of-function mutation in the ␦2 glutamate receptor gene (Grid2) that turns the receptor into a leaky ion channel. The expression of the Lurcher gene in heterozygous (Grid2 Lc/ϩ ) mutants induces the death of almost all Purkinje cells starting from the second postnatal week. Ninety percent of the granule cells and 60-75% of the inferior olivary neurons die because of the loss of their target neurons, the Purkinje cells. The apoptotic nature of the neurodegeneration has been demonstrated previously by the presence of activated caspase-3 and DNA fragmentation. Bax, a pro-apoptotic gene of the Bcl-2 family, has been shown to be involved in developmental neuronal death. To study the role of Bax in Grid2 Lc/ϩ neurodegeneration, double mutants with Grid2 Lc/ϩ mice and Bax knock-out mice (BaxϪ/Ϫ) were generated. Bax deletion had no effect on the death of Purkinje cells and inferior olivary neurons, although a temporary rescue of some Purkinje cells could be detected in P15 Grid2 Lc/ϩ ;BaxϪ/Ϫ animals. From postnatal day 15 (P15) to P60, the number of granule cells in Grid2 Lc/ϩ ;BaxϪ/Ϫmice did not significantly change and was significantly increased compared with the number found in Grid2 Lc/ϩ ;Baxϩ/ϩ mice. Granule cell number in P60 Grid2 Lc/ϩ ;BaxϪ/Ϫ mice corresponded to 70% of the number found in wild-type mice. Our results show that Bax inactivation in Grid2 Lc/ϩ mice does not rescue intrinsic Purkinje cell death or the target-related cell death of olivary neurons, but Bax inactivation does inhibit persistently target-related cell death in cerebellar granule cells.

BH3-only proteins: key regulators of neuronal apoptosis

“…For example, experiments with bim À/À mice have demonstrated that Bim does not play an indispensable role in the cerebellar neurodegeneration that occurs in Lurcher mutant mice, whereas bax inactivation does reduce the death of CGNs, but not Purkinje neurons, in Lurcher mice. 71,72 In the case of bim À/À Lurcher neurons, other BH3-only proteins may substitute for the loss of Bim.…”

Bax ablation prevents dopaminergic neurodegeneration in the 1-methyl- 4-phenyl-1,2,3,6-tetrahydropyridine mouse model of Parkinson's disease

“…In keeping with this, Bax (9) has emerged as a pro-cell death driving force within the central decision point constituted by the Bcl-2 family that modulates the activation of downstream effectors of cell death such as caspases (7). It is also clear that Bax is required for the death of several types of neurons in the peripheral and central nervous systems during both normal development and pathological situations (10)(11)(12)(13)(14)(15)(16)(17)(18). In light of its critical role within the programmed cell death machinery and its importance in neuronal death, Bax appears as a particularly appealing target for therapeutic interventions aimed at hampering neurodegeneration.…”

Glycogen Synthase Kinase-3β Phosphorylates Bax and Promotes Its Mitochondrial Localization during Neuronal Apoptosis

“…Cytosolic cytochrome c then interacts with Apaf-1 and pro-caspase-9 to form a functional apoptosome that ultimately activates downstream executioner caspases (Zou et al, 1999). Many models of neuronal apoptosis occur via this Baxdependent mitochondrial pathway (Cregan et al, 1999;Putcha et al, 1999;Selimi et al, 2000;Vila et al, 2001). Yet despite the prevalence of Bax involvement in neuronal apoptosis, the cellular mechanisms that regulate this Bcl-2 family member, particularly the role of phosphorylation, have not been clearly defined.…”