Paediatric Strategy Forum for medicinal product development in mitogen-activated protein kinase pathway inhibitors: ACCELERATE in collaboration with the European Medicines Agency with participation of the Food and Drug Administration

As the mitogen-activated protein kinase (MAPK) signalling pathway is activated in many paediatric cancers, it is an important therapeutic target. Currently, a range of targeted MAPK pathway inhibitors are being developed in adults. However, MAPK signals through many cascades and feedback loops and perturbing the MAPK pathway may have substantial influence on other pathways as well as normal development. In view of these issues, the ninth Paediatric Strategy Forum focused on MAPK inhibitors. Development of MAPK pathway inhibitors to date has been predominantly driven by adult indications such as malignant melanoma. However, these inhibitors may also target unmet needs in paediatric low-grade gliomas, high-grade gliomas, Langerhans cell histiocytosis, juvenile myelomonocytic leukaemia and several other paediatric conditions. Although MAPK inhibitors have demonstrated activity in paediatric cancer, the response rates and duration of responses needs improvement and better documentation. The rapid development and evaluation of combination approaches, based on a deep understanding of biology, is required to optimise responses and to avoid paradoxical tumour growth and other unintended consequences including severe toxicity. Better inhibitors with higher central nervous systempenetration for primary brain tumours and cancers with a propensity for central nervous system metastases need to be studied to determine if they are more effective than agents currently being used, and the optimum duration of therapy with MAPK inhibition needs to be determined. Systematic and coordinated clinical investigations to inform future treatment strategies with MAPK inhibitors, rather than use outside of clinical trials, are needed to fully assess the risks and benefits of these single agents and combination strategies in both front-line and in the refractory/relapse settings. Platform trials could address the investigation of multiple similar products and combinations. Accelerating the introduction of MAPK inhibitors into front-line paediatric studies is a priority, as is ensuring that these studies generate data appropriate for scientific and regulatory purposes. Early discussions with regulators are crucial, particularly if external controls are considered as randomised control trials in small patient populations can be challenging. Functional end-points specific to the populations in which they are studied, such as visual acuity, motor and neuro psychological function are important, as these outcomes are often more reflective of benefit for lower grade tumours (such as paediatric low-grade glioma and plexiform neurofibroma) and should be included in initial study designs for paediatric low-grade glioma. Early prospective discussions and agreements with regulators are necessary. Long-term follow-up of patients receiving MAPK inhibitors is crucial in view of their prolonged administration and the important involvement of this pathway in normal development. Further rational development, with a detailed understanding of biology of this class of products, is crucial to ensure they provide optimal benefit while minimising toxicity to children and adolescents with cancer.

Olig2 Ablation in Immature Oligodendrocytes Does Not Enhance CNS Myelination and Remyelination

The oligodendrocyte (OL) lineage transcription factor Olig2 is expressed throughout oligodendroglial development and is essential for oligodendroglial progenitor specification and differentiation. It was previously reported that deletion of Olig2 enhanced the maturation and myelination of immature OLs and accelerated the remyelination process. However, by analyzing multiple Olig2 conditional KO mouse lines (male and female), we conclude that Olig2 has the opposite effect and is required for OL maturation and remyelination. We found that deletion of Olig2 in immature OLs driven by an immature OL-expressing Plp1 promoter resulted in defects in OL maturation and myelination, and did not enhance remyelination after demyelination. Similarly, Olig2 deletion during premyelinating stages in immature OLs using Mobp or Mog promoter-driven Cre lines also did not enhance OL maturation in the CNS. Further, we found that Olig2 was not required for myelin maintenance in mature OLs but was critical for remyelination after lysolecithin-induced demyelinating injury. Analysis of genomic occupancy in immature and mature OLs revealed that Olig2 targets the enhancers of key myelination-related genes for OL maturation from immature OLs. Together, by leveraging multiple immature OL-expressing Cre lines, these studies indicate that Olig2 is essential for differentiation and myelination of immature OLs and myelin repair. Our findings raise fundamental questions about the previously proposed role of Olig2 in opposing OL myelination and highlight the importance of using Cre-dependent reporter(s) for lineage tracing in studying cell state progression.SIGNIFICANCE STATEMENT Identification of the regulators that promote oligodendrocyte (OL) myelination and remyelination is important for promoting myelin repair in devastating demyelinating diseases. Olig2 is expressed throughout OL lineage development. Ablation of Olig2 was reported to induce maturation, myelination, and remyelination from immature OLs. However, lineage-mapping analysis of Olig2-ablated cells was not conducted. Here, by leveraging multiple immature OL-expressing Cre lines, we observed no evidence that Olig2 ablation promotes maturation or remyelination of immature OLs. Instead, we find that Olig2 is required for immature OL maturation, myelination, and myelin repair. These data raise fundamental questions about the proposed inhibitory role of Olig2 against OL maturation and remyelination. Our findings highlight the importance of validating genetic manipulation with cell lineage tracing in studying myelination.

Stage-specific control of oligodendrocyte survival and morphogenesis by TDP-43

Generation of oligodendrocytes in the adult brain enables both adaptive changes in neural circuits and regeneration of myelin sheaths destroyed by injury, disease, and normal aging. This transformation of oligodendrocyte precursor cells (OPCs) into myelinating oligodendrocytes requires processing of distinct mRNAs at different stages of cell maturation. Although mislocalization and aggregation of the RNA-binding protein, TDP-43, occur in both neurons and glia in neurodegenerative diseases, the consequences of TDP-43 loss within different stages of the oligodendrocyte lineage are not well understood. By performing stage-specific genetic inactivation of Tardbp in vivo, we show that oligodendrocyte lineage cells are differentially sensitive to loss of TDP-43. While OPCs depend on TDP-43 for survival, with conditional deletion resulting in cascading cell loss followed by rapid regeneration to restore their density, oligodendrocytes become less sensitive to TDP-43 depletion as they mature. Deletion of TDP-43 early in the maturation process led to eventual oligodendrocyte degeneration, seizures, and premature lethality, while oligodendrocytes that experienced late deletion survived and mice exhibited a normal lifespan. At both stages, TDP-43-deficient oligodendrocytes formed fewer and thinner myelin sheaths and extended new processes that inappropriately wrapped neuronal somata and blood vessels. Transcriptional analysis revealed that in the absence of TDP-43, key proteins involved in oligodendrocyte maturation and myelination were misspliced, leading to aberrant incorporation of cryptic exons. Inducible deletion of TDP-43 from oligodendrocytes in the adult central nervous system (CNS) induced the same progressive morphological changes and mice acquired profound hindlimb weakness, suggesting that loss of TDP-43 function in oligodendrocytes may contribute to neuronal dysfunction in neurodegenerative disease.

Myelin remodeling through experience-dependent oligodendrogenesis in the adult somatosensory cortex

The generation of oligodendrocytes in the adult CNS provides a means to adapt the properties of circuits to changes in life experience. However, little is known about the dynamics of oligodendrocytes and the extent of myelin remodeling in the mature brain. Using longitudinal in vivo two photon imaging of oligodendrocytes and their progenitors in the mouse cerebral cortex, we show that myelination is an inefficient and extended process, with half of the final complement of oligodendrocytes generated after four months of age. Oligodendrocytes that successfully integrated formed novel sheaths on unmyelinated and sparsely myelinated axons, and were extremely stable, gradually changing the pattern of myelination. Sensory enrichment robustly increased oligodendrocyte integration, but did not change the length of existing sheaths. This experience-dependent enhancement of myelination in the mature cortex may accelerate information transfer in these circuits and strengthen the ability of axons to sustain activity by providing additional metabolic support.

Cell-type specific differences in promoter activity of the ALS-linked C9orf72 mouse ortholog

A hexanucleotide repeat expansion in the C9orf72 gene is the most common cause of inherited forms of the neurodegenerative disease amyotrophic lateral sclerosis (ALS). Both loss-of-function and gain-of-function mechanisms have been proposed to underlie this disease, but the pathogenic pathways are not fully understood. To better understand the involvement of different cell types in the pathogenesis of ALS, we systematically analyzed the distribution of promoter activity of the mouse ortholog of C9orf72 in the central nervous system. We demonstrate that C9orf72 promoter activity is widespread in both excitatory and inhibitory neurons as well as in oligodendrocytes and oligodendrocyte precursor cells. In contrast, few microglia and astrocytes exhibit detectable C9orf72 promoter activity. Although at a gross level, the distribution of C9orf72 promoter activity largely follows overall cellular density, we found that it is selectively enriched in subsets of neurons and glial cells that degenerate in ALS. Specifically, we show that C9orf72 promoter activity is enriched in corticospinal and spinal motor neurons as well as in oligodendrocytes in brain regions that are affected in ALS. These results suggest that cell autonomous changes in both neurons and glia may contribute to C9orf72-mediated disease, as has been shown for mutations in superoxide dismutase-1 (SOD1).

Transient Opening of the Mitochondrial Permeability Transition Pore Induces Microdomain Calcium Transients in Astrocyte Processes

Astrocytes extend highly branched processes that form functionally isolated microdomains, facilitating local homeostasis by redistributing ions, removing neurotransmitters, and releasing factors to influence blood flow and neuronal activity. Microdomains exhibit spontaneous increases in calcium (Ca²⁺), but the mechanisms and functional significance of this localized signaling are unknown. By developing conditional, membrane-anchored GCaMP3 mice, we found that microdomain activity that occurs in the absence of inositol triphosphate (IP3)-dependent release from endoplasmic reticulum arises through Ca²⁺ efflux from mitochondria during brief openings of the mitochondrial permeability transition pore. These microdomain Ca²⁺ transients were facilitated by the production of reactive oxygen species during oxidative phosphorylation and were enhanced by expression of a mutant form of superoxide dismutase 1 (SOD1 G93A) that causes astrocyte dysfunction and neurodegeneration in amyotrophic lateral sclerosis (ALS). By localizing mitochondria to microdomains, astrocytes ensure local metabolic support for energetically demanding processes and enable coupling between metabolic demand and Ca²⁺ signaling events.

Early white matter abnormalities, progressive brain pathology and motor deficits in a novel knock-in mouse model of Huntington's disease

White matter abnormalities have been reported in premanifest Huntington's disease (HD) subjects before overt striatal neuronal loss, but whether the white matter changes represent a necessary step towards further pathology and the underlying mechanism of these changes remains unknown. Here, we characterized a novel knock-in mouse model that expresses mouse HD gene homolog (Hdh) with extended CAG repeat- HdhQ250, which was derived from the selective breeding of HdhQ150 mice. HdhQ250 mice manifest an accelerated and robust phenotype compared with its parent line. HdhQ250 mice exhibit progressive motor deficits, reduction in striatal and cortical volume, accumulation of mutant huntingtin aggregation, decreased levels of DARPP32 and BDNF and altered striatal metabolites. The abnormalities detected in this mouse model are reminiscent of several aspects of human HD. In addition, disturbed myelination was evident in postnatal Day 14 HdhQ250 mouse brain, including reduced levels of myelin regulatory factor and myelin basic protein, and decreased numbers of myelinated axons in the corpus callosum. Thinner myelin sheaths, indicated by increased G-ratio of myelin, were also detected in the corpus callosum of adult HdhQ250 mice. Moreover, proliferation of oligodendrocyte precursor cells is altered by mutant huntingtin both in vitro and in vivo. Our data indicate that this model is suitable for understanding comprehensive pathogenesis of HD in white matter and gray matter as well as developing therapeutics for HD.