Amyotrophic lateral sclerosis (ALS) is a rapidly progressing neurodegenerative disease, characterized by motor neuron (MN) death, for which there are no truly effective treatments. Here, we describe a new small molecule survival screen carried out using MNs from both wildtype and mutant SOD1 mouse embryonic stem cells. Among the hits we found, kenpaullone had a particularly impressive ability to prolong the healthy survival of both types of MNs that can be attributed to its dual inhibition of GSK3 and HGK kinases. Furthermore, kenpaullone also strongly improved the survival of human MNs derived from ALS patient induced pluripotent stem cells and was more active than either of two compounds, olesoxime and dexpramipexole, that recently failed in ALS clinical trials. Our studies demonstrate the value of a stem cell approach to drug discovery and point to a new paradigm for identification and preclinical testing of future ALS therapeutics.

Neurological diseases and trauma often cause demyelination, resulting in the disruption of axonal function and integrity. Endogenous remyelination promotes recovery, but the process is not well understood because no method exists to definitively distinguish regenerated from preexisting myelin. To date, remyelinated segments have been defined as anything abnormally short and thin, without empirical data to corroborate these morphological assumptions. To definitively identify regenerated myelin, we used a transgenic mouse with an inducible membrane-bound reporter and targeted Cre recombinase expression to a subset of glial progenitor cells after spinal cord injury, yielding remarkably clear visualization of spontaneously regenerated myelin in vivo. Early after injury, the mean length of sheaths regenerated by Schwann cells and oligodendrocytes (OLs) was significantly shorter than control, uninjured myelin, confirming past assumptions. However, OL-regenerated sheaths elongated progressively over 6 mo to approach control values. Moreover, OL-regenerated myelin thickness was not significantly different from control myelin at most time points after injury. Thus, many newly formed OL sheaths were neither thinner nor shorter than control myelin, vitiating accepted dogmas of what constitutes regenerated myelin. We conclude that remyelination, once thought to be static, is dynamic and elongates independently of axonal growth, in contrast to stretch-based mechanisms proposed in development. Further, without clear identification, past assessments have underestimated the extent and quality of regenerated myelin.regeneration | plasticity | internode N ervous system disorders including traumatic injury, stroke, and neurodegenerative diseases such as multiple sclerosis induce loss of myelin and myelinating cells, interrupting signal conduction and depriving axons of trophic support essential for survival (1-4). Postmitotic oligodendrocytes (OLs) do not readily participate in remyelination (5, 6). Instead, glial progenitors, distinguished by expression of the α-receptor for PDGF and the chondroitin sulfate proteoglycan neural/glial antigen 2 (NG2) proliferate following demyelination and differentiate into remyelinating cells within a few weeks (7-10). Regeneration of myelin membranes restores saltatory conduction and supports axonal integrity, leading to partial recovery of function (3,4,11,12). However, remyelination can fail during disease progression, and limited or abnormal myelin regeneration is thought to underlie chronic conduction deficits following trauma (11,13,14). Enhancing or substituting endogenous remyelination via pharmacological intervention or stem/progenitor cell transplantation has been a major, but unrealized, focus of clinical therapy development for decades (15)(16)(17).There is much we do not understand about spontaneous remyelination, including the rate of OL regeneration, whether remyelinating cells select specific phenotypes or morphotypes of axons to ensheathe, and whether the initial number, thicknes...

Remyelination following spinal cord injury (SCI) is thought to be incomplete; demyelination is reported to persist chronically and is proposed as a compelling therapeutic target. Yet most reports do not distinguish between the myelin status of intact axons and injury-severed axons whose proximal stumps persist but provide no meaningful function. We previously found full remyelination of spared, intact rubrospinal axons caudal to the lesion in chronic mouse SCI. However, the clinical concept of chronically demyelinated spared axons remains controversial. Since mouse models may have limitations in clinical translation, we asked whether the capacity for full remyelination is conserved in clinically relevant chronic rat SCI. We determined myelin status by examining paranodal protein distribution on anterogradely labeled, intact corticospinal and rubrospinal axons throughout the extent of the lesion. Demyelination was evident on proximal stumps of severed axons, but not on intact axons. For the first time, we demonstrate that a majority of intact axons exhibit remyelination (at least one abnormally short internode, < 100 μm). Remarkably, shortened internodes were significantly concentrated at the lesion epicenter and individual axons were thinned by 23% compared to their rostral and caudal zones. Mathematical modeling predicted a 25% decrease in conduction velocity at the lesion epicenter due to short internodes and axonal thinning. In conclusion, we do not find a large chronically demyelinated population to target with remyelination therapies. Interventions may be better focused on correcting structural or molecular abnormalities of regenerated myelin.