Lecture 4: Multiple Sclerosis and OPCs

What is MS?

a neurological disorder characterized by demyelination of critical brain regions

How many people worldwide suffer from MS?

over 2.8 million people

What is the percentage of relapsing-remitting MS and primary progressive MS cases out of the total amount?

• relapsing-remitting MS: 85-90%

• primary progressive MS: 10-15%

When does MS most commonly develop?

over 60 years of age

What is MS caused by?

a combination of genetic predisposition, environmental factors (chronic vitamin D deficiency, Epstein-Barr virus infection), and autoimmunity

How are MS symptoms managed?

a combination of disease-modifying treatments, relapse and comorbidity management, symptom relief and psychological support, and rehabilitative care with lifestyle changes

True or False: As an autoimmune disorder, many disease-modifying therapeutic drugs target inflammatory pathways, which produce the brain microenvironment conducive to MS pathology (demyelination with concomitant cognitive decline).

true

True or False: A goal of many MS treatments is to prevent disease relapse (in cases of relapsing-remitting MS) and to improve patient quality-of-life.

true

True or False: Although disease-modifying drugs are useful for alleviating many symptoms of MS, therapies that safely and effectively promote remyelination remain elusive.

true

What is Myrf?

myelin regulatory factor; a transcription factor required for OPC differentiation

True or False: Generation of new oligodendrocytes from the OPCs is critical for myelin deposition to CNS axons. However, most differentiating cells die by apoptosis.

true

True or False: The mammalian brain exhibits a notable ability to recover from loss of oligodendrocytes through injury or disease by producing new oligodendrocytes to regenerate myelin sheaths by rapid proliferation and differentiation.

true

What is impaired in MS?

myelin plasticity

Contrast healthy and impaired myelin repair and plasticity in MS.

• healthy: OPC differentiation to oligodendrocytes induces myelin deposition, activity-dependent myelin modification, and myelin repair

• impaired: OPCs cannot be matured in MS, impairing myelin production, repair and plasticity

True or False: There is persistent demyelination, astroglia proliferation, glial scar, and oligodendrocytes depletion in MS.

true

What is known in MS research? What is unknown or gap in the field of MS reasearch?

• known: OPCs are broadly distributed throughout the brain during development and into adulthood, taking part in initial myelin deposition, activity-dependent myelin modification, and myelin repair

• unknown: the mechanisms of OPC differentiation remains elusive

Why does the knowledge gap in OPC differentiation mechanism need to be closed?

understanding this differentiation process may provide the means to reassert proper myelination programs to enhance the recovery of myelin lost through injury, normal aging, or disease such as MS

What was the central hypothesis/major goal of the paper?

• there is no central hypothesis in this paper

• the major goal is to determine the differentiation dynamics of OPCs

  • identify novel biomarkers for OPC differentiation

  • examine how development, demyelination, aging, and inflammation affect OPC differentiation to oligodendrocytes

What were the methods to achieve the goal?

• to identify biomarkers for OPC differentiation: authors analyzed scRNA seq data of oligodendrocytes isolated from the adult brain

• to verify the potential biomarkers for the OPC differentiation: authors performed immunostaining of brain slices

• to test the function of the potential biomarkers in the OPC differentiation: authors used inducible transgenic mice to do OPC lineage tracing and conditionally delete the genes of interest; then do in vivo longitudinal time-lapse imaging of OPC differentiation attempts

• IDENTIFY biomarkers for OPC differentiation: scRNA seq data of oligos

• VERIFY biomarkers for OPC differentiation: immunostaining

• TEST the function of biomarkers: transgenic mice and lineage tracing

Can we identify novel markers of differentiating OPCs?

• single cell RNA-seq of OPCs, NFOs, mature OLs

• expression of chondroitin sulfate proteoglycans (CSPGs) is high in OPCs and NFOs and low in mature OLs

Can we confirm if OPCs produce CSPG?

yes, with an anti-chondroitin sulfate antibody (CS56)

Describe what was found with the confirmation that OPCs produce CSPG.

• CSPG expression in the somatosensory cortex shows dandelion clock-like structures (DACS) of perineuronal nets

• generation of transgenic mice in which the bacterial enzyme chondroitinase ABC can be inducibly expressed by doxycycline (DOX) withdrawal

• ChABC selectively digests CSPG carbohydrate chains

• selective and reversible ablation of CSPGs by inducing the expression of ChABC confirms labeling specificity of immunostaining

Do CSPGs label cells of the oligodendrocyte lineage?

• NG2 is highly expressed by OPCs (thus is an OPC marker)

• CS56 (DACS)+ cells show NG2 immunostaining but much weaker

• to visualize OPC differentiation, the Pdgfra-CreER, RCE mice express enhanced fluorescent protein (EGFP) in OPCs and their differentiation stages

• CS56+ cells are derived from OPCs (EGFP+) and are NFOs but not OLs

What kind of cells does CS56 label?

• newly formed oligodendrocytes

• oligodendrocyte precursor cells

What is BCAS1?

a gene that is up-regulated at the NFO stage

Does aging slow oligodendrocyte differentiation?

• aging reduces DACS and BCAS+ NFOs

• aging reduces differentiation attempts, successful integrations, and proliferative capacity of OPCs

Does demyelination affect OPC differentiation capacity?

• cuprizone induces rapid death of oligodendrocytes—leading to demyelination

• no, surprisingly the expression of DACS and differentiation attempts remain constant following cuprizone-induced demyelination

How is neuroinflammation induced?

a high dose of lipopolysaccharide (LPS) induces rapid microglia activation and production of inflammatory cytokines in the brain

Does neuroinflammation affect OPC differentiation?

• LPS has no effect on overall OPC density

• LPS reduces DACS expression (a marker for differentiation)

What is indicative of microglia hypertrophy?

microglia activation

What are the two key results?

• new cell type

• connection to MS

Did the authors prove their hypothesis or achieve their goal?

• the expression of CSPGs were found to be transcriptionally upregulated in oligodendrocyte during differentiation attempts

• subsequent experiments demonstrated that immunohistochemical staining of CS56 labels a population of newly formed oligodendrocytes

• YES

What is the significance of this paper?

• identified a new population of constituvely differentiating oligodendrocytes (newly formed oligodendrocytes) in the mammalian CNS that supply myelin and they display an enriched expression of CS56

• aging and inflammation reduces these populations but in the presence of acute demyelination, their differentiation capacity does not change

What is the future direction of this paper?

• are these CS56 cells dysregulated in MS?

  • recent epidemiological studies revealed that the age of onset for MS is increasing, could extending the life of these CS56+ oligos improve disease outcomes, especially for older adults?

• is there a way to influence the lifelong supply of OPCs to enter this constitutive differentiation state