Aging is a major risk factor for both genetic and sporadic neurodegenerative disorders. However, it is unclear how aging interacts with genetic predispositions to promote neurodegeneration. Here, we investigate how partial loss of function of TBK1, a major genetic cause for amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) comorbidity, leads to age-dependent neurodegeneration. We show that TBK1 is an endogenous inhibitor of RIPK1 and the embryonic lethality of Tbk1 mice is dependent on RIPK1 kinase activity. In aging human brains, another endogenous RIPK1 inhibitor, TAK1, exhibits a marked decrease in expression. We show that in Tbk1 mice, the reduced myeloid TAK1 expression promotes all the key hallmarks of ALS/FTD, including neuroinflammation, TDP-43 aggregation, axonal degeneration, neuronal loss, and behavior deficits, which are blocked upon inhibition of RIPK1. Thus, aging facilitates RIPK1 activation by reducing TAK1 expression, which cooperates with genetic risk factors to promote the onset of ALS/FTD.

It is thought that spinal cord injury triggers scar formation with little axon regeneration in mammals 1 – 4 . Here we report that in neonatal mice, a crush injury to the spinal cord leads to a scar-free healing that permits the growth of long projecting axons through the lesion. Depletion of microglia in neonates disrupts such healing and stalls axon regrowth, suggesting a critical role for microglia in orchestrating the injury response. Using single cell RNA-sequencing and functional analyses, we discovered that neonatal microglia undergo a transient activation and play at least two critical roles in scar-free healing. First, they transiently secrete fibronectin and its binding proteins, to form extracellular matrix bridges that ligate the severed ends. Second, neonatal, but not adult, microglia express a number of extracellular and intracellular peptidase inhibitors, along with other molecules involved in inflammatory resolution. Strikingly, upon transplantation into adult spinal cord lesions, both adult microglia treated with peptidases inhibitors and neonatal microglia significantly improve healing and axon regrowth. Together, our results reveal the cellular and molecular basis underlying the nearly complete recovery after spinal cord injury in neonatal mice, pointing to potential strategies to facilitate scar-free healing in the adult mammalian nervous system.

RIPK1 is a critical mediator of cell death and inflammation downstream of TNFR1 upon stimulation by TNFα, a potent proinflammatory cytokine involved in a multitude of human inflammatory and degenerative diseases. RIPK1 contains an N-terminal kinase domain, an intermediate domain, and a C-terminal death domain (DD). The kinase activity of RIPK1 promotes cell death and inflammation. Here, we investigated the involvement of RIPK1-DD in the regulation of RIPK1 kinase activity. We show that a charge-conserved mutation of a lysine located on the surface of DD (K599R in human RIPK1 or K584R in murine RIPK1) blocks RIPK1 activation in necroptosis and RIPK1-dependent apoptosis and the formation of complex II. knockin mutant cells are resistant to RIPK1 kinase-dependent apoptosis and necroptosis. The resistance of K584R cells, however, can be overcome by forced dimerization of RIPK1. Finally, we show that the K584R RIPK1 knockin mutation protects mice against TNFα-induced systematic inflammatory response syndrome. Our study demonstrates the role of RIPK1-DD in mediating RIPK1 dimerization and activation of its kinase activity during necroptosis and RIPK1-dependent apoptosis.