Spinal Cord Injury Results in Chronic Mechanical Stiffening.

Gliosis and fibrosis after spinal cord injury (SCI) lead to formation of a scar that is thought to present both molecular and mechanical barriers to neuronal regeneration. The scar consists of a meshwork of reactive glia and deposited, cross-linked, extracellular matrix (ECM) that has long been assumed to present a mechanically "stiff" blockade. However, remarkably little quantitative information is available about the rheological properties of chronically injured spinal tissue. In this study we utilize atomic force microscopy microindentation to provide quantitative evidence of chronic mechanical stiffening after SCI. Using the results of this tissue characterization, we assessed the sensitivity of both mouse and human astrocytes in vitro and determined that they are exquisitely mechanosensitive within the relevant range of substrate stiffness observed in the injured/uninjured spinal cord. We then utilized a novel immune modifying nanoparticle (IMP) treatment as a tool to reveal fibrotic scarring as one of the key drivers of mechanical stiffening after SCI in vivo. We also demonstrate that glial scar-forming astrocytes form a highly aligned, anisotropic network of glial fibers after SCI, and that IMP treatment mitigates this pathological alignment. Taken together, our results identify chronic mechanical stiffening as a critically important aspect of the complex lesion milieu after SCI that must be considered when assessing and developing potential clinical interventions for SCI.

Neuronal apolipoprotein E4 increases cell death and phosphorylated tau release in alzheimer disease.

OBJECTIVE: The apolipoprotein E (APOE) E4 isoform is the strongest genetic risk factor for sporadic Alzheimer disease (AD). Although APOE is predominantly expressed by astrocytes in the central nervous system, neuronal expression of APOE is of increasing interest in age-related cognitive impairment, neurological injury, and neurodegeneration. Here, we show that endogenous expression of E4 in stem-cell-derived neurons predisposes them to injury and promotes the release of phosphorylated tau. METHODS: Induced pluripotent stem cells from 2 unrelated AD patients carrying the E4 allele were corrected to the E3/E3 genotype with the CRISPR/Cas9 system and differentiated into pure cultures of forebrain excitatory neurons without contamination from other cells types. RESULTS: Compared to unedited E4 neurons, E3 neurons were less susceptible to ionomycin-induced cytotoxicity. Biochemically, E4 cells exhibited increased tau phosphorylation and ERK1/2 phosphoactivation. Moreover, E4 neurons released increased amounts of phosphorylated tau extracellularly in an isoform-dependent manner by a heparin sulfate proteoglycan-dependent mechanism. INTERPRETATION: Our results demonstrate that endogenous expression of E4 by stem-cell-derived forebrain excitatory neurons predisposes neurons to calcium dysregulation and ultimately cell death. This change is associated with increased cellular tau phosphorylation and markedly enhanced release of phosphorylated tau. Importantly, these effects are independent of glial APOE. These findings suggest that E4 accelerates spreading of tau pathology and neuron death in part by neuron-specific, glia-independent mechanisms. Ann Neurol 2019;85:726-739.

BMP-Responsive Protease HtrA1 Is Differentially Expressed in Astrocytes and Regulates Astrocytic Development and Injury Response.

Astrocytes perform a wide array of physiological functions, including structural support, ion exchange, and neurotransmitter uptake. Despite this diversity, molecular markers that label subpopulations of astrocytes are limited, and mechanisms that generate distinct astrocyte subtypes remain unclear. Here we identified serine protease high temperature requirement A 1 (HtrA1), a bone morphogenetic protein 4 signaling regulated protein, as a novel marker of forebrain astrocytes, but not of neural stem cells, in adult mice of both sexes. Genetic deletion of HtrA1 during gliogenesis accelerates astrocyte differentiation. In addition, ablation of HtrA1 in cultured astrocytes leads to altered chondroitin sulfate proteoglycan expression and inhibition of neurite extension, along with elevated levels of transforming growth factor-ß family proteins. Brain injury induces HtrA1 expression in reactive astrocytes, and loss of HtrA1 leads to an impairment in wound closure accompanied by increased proliferation of endothelial and immune cells. Our findings demonstrate that HtrA1 is differentially expressed in adult mouse forebrain astrocytes, and that HtrA1 plays important roles in astrocytic development and injury response.SIGNIFICANCE STATEMENT Astrocytes, an abundant cell type in the brain, perform a wide array of physiological functions. Although characterized as morphologically and functionally diverse, molecular markers that label astrocyte subtypes or signaling pathways that lead to their diversity remain limited. Here, after examining the expression profile of astrocytes generated in response to bone morphogenetic protein signaling, we identify high temperature requirement A 1 (HtrA1) as an astrocyte-specific marker that is differentially expressed in distinct adult mouse brain regions. HtrA1 is a serine protease that has been linked to cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy, a small blood vessel disease in humans. Understanding the role of HtrA1 during development and after injury will provide insights into how distinct astrocyte populations are generated and their unique roles in injury and disease.

Periodontal ligament cells cultured under steady-flow environments demonstrate potential for use in heart valve tissue engineering.

A major drawback of mechanical and prosthetic heart valves is their inability to permit somatic growth. By contrast, tissue-engineered pulmonary valves potentially have the capacity to remodel and integrate with the patient. For this purpose, adult stem cells may be suitable. Previously, human periodontal ligament cells (PDLs) have been explored as a reliable and robust progenitor cell source for cardiac muscle regeneration (Pelaez, D. Electronic Thesis and Dissertation Database, Coral Gables, FL, May 2011). Here, we investigate the potential of PDLs to support the valve lineage, specifically the concomitant differentiation to both endothelial cell (EC) and smooth muscle cell (SMC) types. We were able to successfully promote PDL differentiation to both SMC and EC phenotypes through a combination of stimulatory approaches using biochemical and mechanical flow conditioning (steady shear stress of 1 dyne/cm(2)), with flow-based mechanical conditioning having a predominant effect on PDL differentiation, particularly to ECs; in addition, strong expression of the marker FZD2 and an absence of the marker MLC1F point toward a unique manifestation of smooth muscle by PDLs after undergoing steady-flow mechanical conditioning alone, possible by only the heart valve and pericardium phenotypes. It was also determined that steady flow (which was performed using a physiologically relevant [for heart valves] magnitude of ~5-6 dynes/cm(2)) augmented the synthesis of the extracellular matrix collagen proteins. We conclude that under steady-flow dynamic culture environments, human PDLs can differentiate to heterogeneous cell populations that are relevant to heart valve tissue engineering. Further exploration of human PDLs for this purpose is thus warranted.