Blocking the mitochondrial apoptotic pathway preserves motor neuron viability and function in a mouse model of amyotrophic lateral sclerosis

Abstract

Apoptosis of motor neurons is a well-documented feature in amyotrophic lateral sclerosis (ALS) and related motor neuron diseases (MNDs). However, the role of apoptosis in the pathogenesis of these diseases remains unresolved. One possibility is that the affected motor neurons only succumb to apoptosis once they have exhausted functional capacity. If true, blocking apoptosis should confer no therapeutic benefit. To directly investigate this idea, we tested whether tissue-specific deletion in the mouse CNS of BCL2-associated X protein (BAX) and BCL2-homologous antagonist/killer (BAK), 2 proapoptotic BCL-2 family proteins that together represent an essential gateway to the mitochondrial apoptotic pathway, would protect against motor neuron degeneration. We found that neuronal deletion of Bax and Bak in a mouse model of familial ALS not only halted neuronal loss, but prevented axonal degeneration, symptom onset, weight loss, and paralysis and extended survival. These results show that motor neurons damaged in ALS activate the mitochondrial apoptotic pathway early in the disease process and that apoptotic signaling directly contributes to neuromuscular degeneration and neuronal dysfunction. Hence, inhibiting apoptosis upstream of mitochondrial permeabilization represents a possible therapeutic strategy for preserving functional motor neurons in ALS and other MNDs.

ATF3 expression improves motor function in the ALS mouse model by promoting motor neuron survival and retaining muscle innervation

“…Mechanisms that act to protect only one compartment, either the motor neuron soma or the axons, do not seem sufficient to preserve a functional motor unit in ALS (42)(43)(44)(45)(46). The importance of both maintaining axonal connectivity with the muscle and protecting the motor neurons from cell death for preservation of a functional motor unit is clearly illustrated in this study.…”

“…Three different experimental manipulations can delay motor neuron soma death in SOD1 mutant ALS mice: overexpression of the antiapoptotic protein Bcl-2, KO of the proapoptotic Bax protein, and double KO of BAX and BAC (42)(43)(44). However, even in the BAX/BAC double KO mice (42), in which a substantial delay in motor neurons cell death is observed, axonal integrity is not preserved to the same degree, and the motor axon terminals continue to gradually die back from the NMJ. Despite the substantial preservation of motor neuron soma achieved in these mouse model experiments, motor neuron survival alone does not prevent axons from degenerating.…”

Species-dependent neuropathology in transgenic SOD1 pigs

“…Real-time PCR of transgene copy numbers revealed that transgenic pigs carried various numbers (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) of the transgene (Supplementary information, Table S1). Using immunohistochemistry staining with an antibody specific to hSOD1, we could confirm the expression of transgenic hSOD1 in the lumbar spinal cords of a stillborn transgenic pig, but not in a wild-type one ( Figure Figure 1 Generation of hSOD1 (G93A) transgenic pigs.…”

“…These transgenic SOD1 mouse models carry a greatly elevated protein load compared with human ALS tissues, and these animals exhibit some phenotypic features, such as intracytoplasmic vacuoles derived from the mitochondria and the endoplasmic reticulum, which are rarely found in human ALS tissues [11][12][13]. In terms of the mechanisms of neuronal death in ALS, most evidence from mouse models suggests that apoptosis may play a crucial role in the death of motor neurons [14][15][16]; however, conclusive evidence of apoptosis has been difficult to detect in humans [17][18][19][20]. In addition, intranuclear inclusions were seen in the brains of ALS patient with SOD1 mutations [21][22][23], which have nevertheless not been found in transgenic mutant SOD1 mouse models.…”

The BCL-2 family protein Bid is critical for pro-inflammatory signaling in astrocytes

“…Bax was 24 shown to accumulate in mitochondria in animal models of ALS (Guegan et al, 2001), and 25 deletion of the bax gene in mtSOD1 transgenic mice inhibited motoneuron death (Gould et 26 al., 2006). Recently it was shown that conditional, combined deletion of bax and bak 1 potently delayed disease onset and progression in the SOD1 G93A mouse model of ALS 2 (Reyes et al, 2010). BH3-only proteins responsible for the activation of Bax (and 3 potentially Bak) in animal models of ALS were also recently identified.…”