Michael B. Steketee

Immunomodulatory approaches to CNS injury: extracellular matrix and exosomes from extracellular matrix conditioned macrophages.

After central nervous system (CNS) injury, a pro-inflammatory, innate immune response contributes to permanently lost neuronal function by promoting changes in the micro-environment and extracellular matrix (ECM) that lead to CNS neuronal degeneration and death and permanent scarring. This pro-inflammatory immune response is largely due to activated resident microglia and infiltrating macrophages adopting a predominantly pro-inflammatory, M1-like, phenotype. Activated, M1-like, microglia and macrophages release additional pro-inflammatory factors, including cytokines and microvesicles like exosomes, that recruit additional cell types, including activated microglia and astrocytes and naive blood macrophages, M0, that enter the injured tissue and polarize toward M1-like phenotypes that can lead to secondary tissue damage and increased scarring (Figure 1A). After the initial inflammatory response resolves, M1-like macrophages and microglia persist due to pro-inflammatory cytokine signaling cascades that act by both autocrine and paracrine signaling mechanisms to maintain pro-inflammatory signaling, ultimately leading to permanent scarring. Scar tissue also appears to promote persistent inflammatory signaling. Thus, a logical approach to treating CNS injuries is to develop immunomodulatory strategies that interrupt cyclical pro-inflammatory signaling cascades while promoting anti-inflammatory, M2-like, signaling, ultimately altering the default healing response in the CNS from tissue destruction and scarring toward positive tissue and ECM remodeling.

An Elastomeric Polymer Matrix, PEUU-Tac, Delivers Bioactive Tacrolimus Transdurally to the CNS in Rat

Central nervous system (CNS) neurons fail to regrow injured axons, often resulting in permanently lost neurologic function. Tacrolimus is an FDA-approved immunosuppressive drug with known neuroprotective and neuroregenerative properties in the CNS. However, tacrolimus is typically administered systemically and blood levels required to effectively treat CNS injuries can lead to lethal, off-target organ toxicity. Thus, delivering tacrolimus locally to CNS tissues may provide therapeutic control over tacrolimus levels in CNS tissues while minimizing off-target toxicity. Herein we show an electrospun poly(ester urethane) urea and tacrolimus elastomeric matrix (PEUU-Tac) can deliver tacrolimus trans-durally to CNS tissues. In an acute CNS ischemia model in rat, the optic nerve (ON) was clamped for 10s and then PEUU-Tac was used as an ON wrap and sutured around the injury site. Tacrolimus was detected in PEUU-Tac wrapped ONs at 24 h and 14 days, without significant increases in tacrolimus blood levels. Similar to systemically administered tacrolimus, PEUU-Tac locally decreased glial fibrillary acidic protein (GFAP) at the injury site and increased growth associated protein-43 (GAP-43) expression in ischemic ONs from the globe to the chiasm, consistent with decreased astrogliosis and increased retinal ganglion cell (RGC) axon growth signaling pathways. These initial results suggest PEUU-Tac is a biocompatible elastic matrix that delivers bioactive tacrolimus trans-durally to CNS tissues without significantly increasing tacrolimus blood levels and off-target toxicity.

Fetal extracellular matrix nerve wraps locally improve peripheral nerve remodeling after complete transection and direct repair in rat

In peripheral nerve (PN) injuries requiring surgical repair, as in PN transection, cellular and ECM remodeling at PN epineurial repair sites is hypothesized to reduce PN functional outcomes by slowing, misdirecting, or preventing axons from regrowing appropriately across the repair site. Herein this study reports on deriving and analyzing fetal porcine urinary bladder extracellular matrix (fUB-ECM) by vacuum assisted decellularization, fabricating fUBM-ECM nerve wraps, and testing fUB-ECM nerve wrap biocompatibility and bioactivity in a trigeminal, infraorbital nerve (ION) branch transection and direct end-to-end repair model in rat. FUB-ECM nerve wraps significantly improved epi- and endoneurial organization and increased both neovascularization and growth associated protein-43 (GAP-43) expression at PN repair sites, 28-days post surgery. However, the number of neurofilament positive axons, remyelination, and whisker-evoked response properties of ION axons were unaltered, indicating improved tissue remodeling per se does not predict axon regrowth, remyelination, and the return of mechanoreceptor cortical signaling. This study shows fUB-ECM nerve wraps are biocompatible, bioactive, and good experimental and potentially clinical devices for treating epineurial repairs. Moreover, this study highlights the value provided by precise, analytic models, like the ION repair model, in understanding how PN tissue remodeling relates to axonal regrowth, remyelination, and axonal response properties.

Extracellular Vesicles: Biomarkers, Therapeutics, and Vehicles in the Visual System

PurposeWe discuss recent advances in extracellular vesicle (EV) technology as biomarkers, therapeutics, and drug delivery vehicles in the visual system with an emphasis on the retina.Recent FindingsRetinal cell-type specific EVs can be detected in the blood and in the aqueous humor and EV miRNA cargoes can be used diagnostically to predict retinal disease progression. Studies have now shown EVs can deliver bioactive miRNA and AAV cargoes to the inner retinal cell layers and, in some models, improve retinal ganglion cell (RGC) survival and axon regeneration.SummaryEV molecular profiles and cargoes are attractive biomarkers for retinal and optic nerve disease and trauma and EVs offer a safe and tunable platform for delivering therapies to ocular tissues. However, EVs are heterogeneous by nature with variable lipid membranes, cargoes, and biologic effects, warranting stringent characterization to understand how heterogeneous EV populations modulate positive tissue remodeling.

Light-evoked activity and BDNF regulate mitochondrial dynamics and mitochondrial localized translation

Mitochondria coordinate diverse functions within neurites, including signaling events for axonal maintenance, and degeneration. However, less is known about the role of mitochondria in axon development and maturation. Here we find that in maturing retinal ganglion cells (RGCs) in vivo, axonal mitochondria increase in size, number, and total area throughout development. We demonstrate through multiple approaches in vivo that the mechanism underlying these mitochondrial changes are dependent on eye opening and associated neuronal activity, which can be mimicked by brain derived neurotrophic factor (BDNF). We report downstream gene and protein expression changes consistent with mitochondrial biogenesis and energetics pathways, and present evidence that the associated transcripts are localized and translated at mitochondria within axons in an activity-dependent manner. Together these data support a novel model for mitochondrial-localized translation in support of intra-axonal mitochondrial dynamics and axonal maturation.

Correction to: Extracellular Vesicles: Biomarkers, Therapeutics, and Vehicles in the Visual System

The article Extracellular Vesicles: Biomarkers, Therapeutics, and Vehicles in the Visual System, written by Yolandi van der Merwe and Michael B. Steketee, was originally published Online First without open access.