Wild-type microglia extend survival in PU.1 knockout mice with familial amyotrophic lateral sclerosis

TLDR

Familial ALS is most commonly caused by dominant SOD1 mutations, and recent evidence suggests that glial cells may contribute to motoneuron injury in this disease. The study aimed to determine whether mutant SOD1 expression in CNS microglia, motoneurons, or astroglia contributes to motoneuron injury by using PU.1‑/‑ mice lacking myeloid cells and bone‑marrow transplantation or breeding with mSOD1 G93A mice. PU.1‑/‑ mice were reconstituted with donor bone marrow to generate microglia lacking PU.1, and then crossed with mSOD1 G93A mice to produce animals with mutant SOD1 expression in specific CNS cell types. Wild‑type donor microglia transplanted into PU.1‑/‑ mice slowed motoneuron loss and extended survival, whereas mutant SOD1‑expressing microglia were more neurotoxic, producing higher reactive oxygen species and causing greater neuronal death, demonstrating that mutant SOD1 drives microglial activation and motoneuron injury.

Abstract

The most common inherited form of amyotrophic lateral sclerosis (ALS), a neurodegenerative disease affecting adult motoneurons, is caused by dominant mutations in the ubiquitously expressed Cu 2+ /Zn 2+ superoxide dismutase (SOD1). Recent studies suggest that glia may contribute to motoneuron injury in animal models of familial ALS. To determine whether the expression of mutant SOD1 (mSOD1 G93A ) in CNS microglia contributes to motoneuron injury, PU.1 −/− mice that are unable to develop myeloid and lymphoid cells received bone marrow transplants resulting in donor-derived microglia. Donor-derived microglia from mice overexpressing mSOD1 G93A , an animal model of familial ALS, transplanted into PU.1 −/− mice could not induce weakness, motoneuron injury, or an ALS-like disease. To determine whether expression of mSOD1 G93A in motoneurons and astroglia, as well as microglia, was required to produce motoneuron disease, PU.1 −/− mice were bred with mSOD1 G93A mice. In mSOD1 G93A /PU.1 −/− mice, wild-type donor-derived microglia slowed motoneuron loss and prolonged disease duration and survival when compared with mice receiving mSOD1 G93A expressing cells or mSOD1 G93A mice. In vitro studies confirmed that wild-type microglia were less neurotoxic than similarly cultured mSOD1 G93A microglia. Compared with wild-type microglia, mSOD1 G93A microglia produced and released more superoxide and nitrite+nitrate, and induced more neuronal death. These data demonstrate that the expression of mSOD1 G93A results in activated and neurotoxic microglia, and suggests that the lack of mSOD1 G93A expression in microglia may contribute to motoneuron protection. This study confirms the importance of microglia as a double-edged sword, and focuses on the importance of targeting microglia to minimize cytotoxicity and maximize neuroprotection in neurodegenerative diseases.