The bright and the dark side of myelin plasticity: Neuron-glial interactions in health and disease

Transient Upregulation of Procaspase-3 during Oligodendrocyte Fate Decisions

Oligodendrocytes are generated throughout life and in neurodegenerative conditions from brain resident oligodendrocyte precursor cells (OPCs). The transition from OPC to oligodendrocyte involves a complex cascade of molecular and morphological states that position the cell to make a fate decision to integrate as a myelinating oligodendrocyte or die through apoptosis. Oligodendrocyte maturation impacts the cell death mechanisms that occur in degenerative conditions, but it is unclear if and how the cell death machinery changes as OPCs transition into oligodendrocytes. Here, we discovered that differentiating oligodendrocytes transiently upregulate the zymogen procaspase-3 in both female and male mice, equipping these cells to make a survival decision during differentiation. Pharmacological inhibition of caspase-3 decreases oligodendrocyte density, indicating that procaspase-3 upregulation is linked to successful oligodendrocyte generation. Moreover, using procaspase-3 as a marker, we show that oligodendrocyte differentiation continues in the aging cortex and white matter. Taken together, our data establish procaspase-3 as a differentiating oligodendrocyte marker and provide insight into the underlying mechanisms occurring during the decision to integrate or die.

Transient upregulation of procaspase-3 during oligodendrocyte fate decisions

Oligodendrocytes are generated throughout life and in neurodegenerative conditions from brain resident oligodendrocyte precursor cells (OPCs). The transition from OPC to oligodendrocyte involves a complex cascade of molecular and morphological states that position the cell to make a fate decision to integrate as a myelinating oligodendrocyte or die through apoptosis. Oligodendrocyte maturation impacts the cell death mechanisms that occur in degenerative conditions, but it is unclear if and how the cell death machinery changes as OPCs transition into oligodendrocytes. Here, we discovered that differentiating oligodendrocytes transiently upregulate the zymogen procaspase-3, equipping these cells to make a survival decision during differentiation. Pharmacological inhibition of caspase-3 decreases oligodendrocyte density, indicating that procaspase-3 upregulation promotes differentiation. Moreover, using procaspase-3 as a marker, we show that oligodendrocyte differentiation continues in the aging cortex and white matter. Taken together, our data establish procaspase-3 as a differentiating oligodendrocyte marker and provide insight into the underlying mechanisms occurring during the decision to integrate or die.

Tackling myelin deficits in neurodevelopmental disorders using drug delivery systems

Interest in myelin and its roles in almost all brain functions has been greatly increasing in recent years, leading to countless new studies on myelination, as a dominant process in the development of cognitive functions. Here, we explore the unique role myelin plays in the central nervous system and specifically discuss the results of altered myelination in neurodevelopmental disorders. We present parallel developmental trajectories involving myelination that correlate with the onset of cognitive impairment in neurodevelopmental disorders and discuss the key challenges in the treatment of these chronic disorders. Recent developments in drug repurposing and nano/micro particle-based therapies are reviewed as a possible pathway to circumvent some of the main hurdles associated with early intervention, including patient's adherence and compliance, side effects, relapse, and faster route to possible treatment of these disorders. The strategy of drug encapsulation overcomes drug solubility and metabolism, with the possibility of drug targeting to a specific compartment, reducing side effects upon systemic administration.

ANKS1B encoded AIDA-1 regulates social behaviors by controlling oligodendrocyte function

Heterozygous deletions in the ANKS1B gene cause ANKS1B neurodevelopmental syndrome (ANDS), a rare genetic disease characterized by autism spectrum disorder (ASD), attention deficit/hyperactivity disorder, and speech and motor deficits. The ANKS1B gene encodes for AIDA-1, a protein that is enriched at neuronal synapses and regulates synaptic plasticity. Here we report an unexpected role for oligodendroglial deficits in ANDS pathophysiology. We show that Anks1b-deficient mouse models display deficits in oligodendrocyte maturation, myelination, and Rac1 function, and recapitulate white matter abnormalities observed in ANDS patients. Selective loss of Anks1b from the oligodendrocyte lineage, but not from neuronal populations, leads to deficits in social preference and sensory reactivity previously observed in a brain-wide Anks1b haploinsufficiency model. Furthermore, we find that clemastine, an antihistamine shown to increase oligodendrocyte precursor cell maturation and central nervous system myelination, rescues deficits in social preference in 7-month-old Anks1b-deficient mice. Our work shows that deficits in social behaviors present in ANDS may originate from abnormal Rac1 activity within oligodendrocytes.

Emerging roles of oligodendrocyte precursor cells in neural circuit development and remodeling

Oligodendrocyte precursor cells (OPCs) are non-neuronal brain cells that give rise to oligodendrocytes, glia that myelinate the axons of neurons in the brain. Classically known for their contributions to myelination via oligodendrogenesis, OPCs are increasingly appreciated to play diverse roles in the nervous system, ranging from blood vessel formation to antigen presentation. Here, we review emerging literature suggesting that OPCs may be essential for the establishment and remodeling of neural circuits in the developing and adult brain via mechanisms that are distinct from the production of oligodendrocytes. We discuss the specialized features of OPCs that position these cells to integrate activity-dependent and molecular cues to shape brain wiring. Finally, we place OPCs within the context of a growing field focused on understanding the importance of communication between neurons and glia in the contexts of both health and disease.

Oncohistones in brain tumors: the soil and seed

Recurrent somatic mutations in histone 3 (H3) variants (termed 'oncohistones') have been identified in high-grade gliomas (HGGs) in children and young adults and induce tumorigenesis through disruption of chromatin states. Oncohistones occur with exquisite neuroanatomical specificity and are associated with specific age distribution and epigenome landscapes. Here, we review the known intrinsic ('seed') and the extrinsic ('soil') factors needed for their optimal oncogenic effect and highlight the many unresolved questions regarding their effects on development and crosstalk with the tumor microenvironment. The 'seed and soil' analogy, used to explain tumor metastatic niches, also applies to oncohistones, which mainly thrive and flourish in specific chromatin states during very narrow windows of development, creating exquisite vulnerabilities, which could provide effective therapies for these deadly cancers.

Neurons as stromal drivers of nervous system cancer formation and progression

Similar to their pivotal roles in nervous system development, neurons have emerged as critical regulators of cancer initiation, maintenance, and progression. Focusing on nervous system tumors, we describe the normal relationships between neurons and other cell types relevant to normal nerve function, and discuss how disruptions of these interactions promote tumor evolution, focusing on electrical (gap junctions) and chemical (synaptic) coupling, as well as the establishment of new paracrine relationships. We also review how neuron-tumor communication contributes to some of the complications of cancer, including neuropathy, chemobrain, seizures, and pain. Finally, we consider the implications of cancer neuroscience in establishing risk for tumor penetrance and in the design of future anti-tumoral treatments.

Circadian Control of Glial Cell Homeodynamics

The molecular mechanisms that maintain circadian rhythms in mammalian as well as non-mammalian systems are well documented in neuronal populations but comparatively understudied in glia. Glia are highly dynamic in form and function, and the circadian clock provides broad dynamic ranges for the maintenance of this homeostasis, thus glia are key to understanding the role of circadian biology in brain function. Here, we highlight the implications of the molecular circadian clock on the homeodynamic nature of glia, underscoring the current gap in understanding the role of the circadian system in oligodendroglia lineage cells and subsequent myelination. Through this perspective, we will focus on the intersection of circadian and glial biology and how it interfaces with global circadian rhythm maintenance associated with normative and aberrant brain function.

Neural Signaling in Cancer

Nervous system activity regulates development, homeostasis, and plasticity of the brain as well as other organs in the body. These mechanisms are subverted in cancer to propel malignant growth. In turn, cancers modulate neural structure and function to augment growth-promoting neural signaling in the tumor microenvironment. Approaching cancer biology from a neuroscience perspective will elucidate new therapeutic strategies for presently lethal forms of cancer. In this review, we highlight the neural signaling mechanisms recapitulated in primary brain tumors, brain metastases, and solid tumors throughout the body that regulate cancer progression. Expected final online publication date for the Annual Review of Neuroscience, Volume 45 is July 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Shaping of Regional Differences in Oligodendrocyte Dynamics by Regional Heterogeneity of the Pericellular Microenvironment

Oligodendrocyte precursor cells (OPCs) are glial cells that differentiate into mature oligodendrocytes (OLs) to generate new myelin sheaths. While OPCs are distributed uniformly throughout the gray and white matter in the developing and adult brain, those in white matter proliferate and differentiate into oligodendrocytes at a greater rate than those in gray matter. There is currently lack of evidence to suggest that OPCs comprise genetically and transcriptionally distinct subtypes. Rather, the emerging view is that they exist in different cell and functional states, depending on their location and age. Contrary to the normal brain, demyelinated lesions in the gray matter of multiple sclerosis brains contain more OPCs and OLs and are remyelinated more robustly than those in white matter. The differences in the dynamic behavior of OL lineage cells are likely to be influenced by their microenvironment. There are regional differences in astrocytes, microglia, the vasculature, and the composition of the extracellular matrix (ECM). We will discuss how the regional differences in these elements surrounding OPCs might shape their phenotypic variability in normal and demyelinated states.