Astrocytes protect oligodendrocyte precursor cells via MEK/ERK and PI3K/Akt signaling

Vascular contributions to cognitive impairment and dementia: a statement for healthcare professionals from the american heart association/american stroke association

BACKGROUND AND PURPOSE

This scientific statement provides an overview of the evidence on vascular contributions to cognitive impairment and dementia. Vascular contributions to cognitive impairment and dementia of later life are common. Definitions of vascular cognitive impairment (VCI), neuropathology, basic science and pathophysiological aspects, role of neuroimaging and vascular and other associated risk factors, and potential opportunities for prevention and treatment are reviewed. This statement serves as an overall guide for practitioners to gain a better understanding of VCI and dementia, prevention, and treatment.

METHODS

Writing group members were nominated by the writing group co-chairs on the basis of their previous work in relevant topic areas and were approved by the American Heart Association Stroke Council Scientific Statement Oversight Committee, the Council on Epidemiology and Prevention, and the Manuscript Oversight Committee. The writing group used systematic literature reviews (primarily covering publications from 1990 to May 1, 2010), previously published guidelines, personal files, and expert opinion to summarize existing evidence, indicate gaps in current knowledge, and, when appropriate, formulate recommendations using standard American Heart Association criteria. All members of the writing group had the opportunity to comment on the recommendations and approved the final version of this document. After peer review by the American Heart Association, as well as review by the Stroke Council leadership, Council on Epidemiology and Prevention Council, and Scientific Statements Oversight Committee, the statement was approved by the American Heart Association Science Advisory and Coordinating Committee.

RESULTS

The construct of VCI has been introduced to capture the entire spectrum of cognitive disorders associated with all forms of cerebral vascular brain injury-not solely stroke-ranging from mild cognitive impairment through fully developed dementia. Dysfunction of the neurovascular unit and mechanisms regulating cerebral blood flow are likely to be important components of the pathophysiological processes underlying VCI. Cerebral amyloid angiopathy is emerging as an important marker of risk for Alzheimer disease, microinfarction, microhemorrhage and macrohemorrhage of the brain, and VCI. The neuropathology of cognitive impairment in later life is often a mixture of Alzheimer disease and microvascular brain damage, which may overlap and synergize to heighten the risk of cognitive impairment. In this regard, magnetic resonance imaging and other neuroimaging techniques play an important role in the definition and detection of VCI and provide evidence that subcortical forms of VCI with white matter hyperintensities and small deep infarcts are common. In many cases, risk markers for VCI are the same as traditional risk factors for stroke. These risks may include but are not limited to atrial fibrillation, hypertension, diabetes mellitus, and hypercholesterolemia. Furthermore, these same vascular risk factors may be risk markers for Alzheimer disease. Carotid intimal-medial thickness and arterial stiffness are emerging as markers of arterial aging and may serve as risk markers for VCI. Currently, no specific treatments for VCI have been approved by the US Food and Drug Administration. However, detection and control of the traditional risk factors for stroke and cardiovascular disease may be effective in the prevention of VCI, even in older people.

CONCLUSIONS

Vascular contributions to cognitive impairment and dementia are important. Understanding of VCI has evolved substantially in recent years, based on preclinical, neuropathologic, neuroimaging, physiological, and epidemiological studies. Transdisciplinary, translational, and transactional approaches are recommended to further our understanding of this entity and to better characterize its neuropsychological profile. There is a need for prospective, quantitative, clinical-pathological-neuroimaging studies to improve knowledge of the pathological basis of neuroimaging change and the complex interplay between vascular and Alzheimer disease pathologies in the evolution of clinical VCI and Alzheimer disease. Long-term vascular risk marker interventional studies beginning as early as midlife may be required to prevent or postpone the onset of VCI and Alzheimer disease. Studies of intensive reduction of vascular risk factors in high-risk groups are another important avenue of research.

Cellular mechanisms of neurovascular damage and repair after stroke

The biological processes underlying stroke are complex, and patients have a narrow repertoire of therapeutic opportunities. After the National Institutes of Health (NIH) convened the Stroke Progress Review Group in 2001, stroke research shifted from having a purely neurocentric focus to adopting a more integrated view wherein dynamic interactions between all cell types contribute to function and dysfunction in the brain. This so-called "neurovascular unit" provides a conceptual framework that emphasizes cell-cell interactions between neuronal, glial, and vascular elements. Under normal conditions, signaling within the neurovascular unit helps maintain homeostasis. After stroke, cell-cell signaling is disturbed, leading to pathophysiology. More recently, emerging data now suggest that these cell-cell signaling mechanisms may also mediate parallel processes of neurovascular remodeling during stroke recovery. Because plasticity is a signature feature of the young and developing brain, these concepts may have special relevance to how the pediatric brain responds after stroke.

The ERK2 mitogen-activated protein kinase regulates the timing of oligodendrocyte differentiation

Oligodendrocyte development is tightly controlled by a variety of extracellular growth and differentiation factors. The mitogen-activated protein kinases (MAPKs), ERK1 and ERK2, are critical intracellular signaling molecules important for transducing these extracellular signals. The extracellular signal-regulated kinases (ERKs) are ubiquitously expressed, coordinately regulated, and highly similar, but Erk2 deletion in mice is embryonic lethal whereas Erk1 deletion is not. Several studies have suggested that MAPK signaling is important for oligodendrocyte differentiation, although specific roles for the two ERK isoforms have not been investigated. In this study, we deleted Erk2 in the developing mouse cortex from GFAP-expressing radial glia that generate neurons and oligodendrocytes. In vitro analysis revealed that loss of ERK2 resulted in fewer galactocerebroside-expressing mature oligodendrocytes in cortical cultures. In vivo, a delay in the expression of the myelin protein MBP was observed in the corpus callosum at postnatal day 10 (P10). In contrast, Erk1 deletion did not affect oligodendrocyte differentiation. By P21, MBP expression was restored to wild-type levels, demonstrating that the loss of ERK2 results in a delay but not a complete arrest in the appearance of differentiated oligodendrocytes in vivo. Importantly, both the proliferation and total number of oligodendrocyte progenitor cells (OPCs) appeared normal in the Erk2 conditional knock-out cortex, demonstrating that ERK2 plays a specific role in the timing of forebrain myelination but is not critical for the proliferation or survival of OPCs. Oligodendrocyte-specific deletion of Erk2 also resulted in decreased levels of MBP, indicating a cell-autonomous effect of ERK2 in the oligodendrocyte lineage.

The overlap between neurodegenerative and vascular factors in the pathogenesis of dementia

There is increasing evidence that cerebrovascular dysfunction plays a role not only in vascular causes of cognitive impairment but also in Alzheimer's disease (AD). Vascular risk factors and AD impair the structure and function of cerebral blood vessels and associated cells (neurovascular unit), effects mediated by vascular oxidative stress and inflammation. Injury to the neurovascular unit alters cerebral blood flow regulation, depletes vascular reserves, disrupts the blood-brain barrier, and reduces the brain's repair potential, effects that amplify the brain dysfunction and damage exerted by incident ischemia and coexisting neurodegeneration. Clinical-pathological studies support the notion that vascular lesions aggravate the deleterious effects of AD pathology by reducing the threshold for cognitive impairment and accelerating the pace of the dementia. In the absence of mechanism-based approaches to counteract cognitive dysfunction, targeting vascular risk factors and improving cerebrovascular health offers the opportunity to mitigate the impact of one of the most disabling human afflictions.

The science of stroke: mechanisms in search of treatments

This review focuses on mechanisms and emerging concepts that drive the science of stroke in a therapeutic direction. Once considered exclusively a disorder of blood vessels, growing evidence has led to the realization that the biological processes underlying stroke are driven by the interaction of neurons, glia, vascular cells, and matrix components, which actively participate in mechanisms of tissue injury and repair. As new targets are identified, new opportunities emerge that build on an appreciation of acute cellular events acting in a broader context of ongoing destructive, protective, and reparative processes. The burden of disease is great, and its magnitude widens as a role for blood vessels and stroke in vascular and nonvascular dementias becomes more clearly established. This review then poses a number of fundamental questions, the answers to which may generate new directions for research and possibly new treatments that could reduce the impact of this enormous economic and societal burden.