Novel Insight into Regulation of Glymphatic Flow with Implications for Traumatic Brain Injury

Thesis (Ph.D.)--University of Rochester. School of Medicine & Dentistry. Dept. of Pathology and Laboratory Medicine, 2017.The glymphatic system is a pathway for CSF-ISF exchange responsible for brain waste clearance, and its dysfunction has been implicated in a variety of CNS diseases, including traumatic brain injury (TBI). While bulk fluid flow through this pathway is modulated by cerebral arterial pulsatility, state of consciousness, and even head position, at present, none of these regulatory elements have demonstrated an ability to modify neurologic disease. ISF is secreted by the cerebrovascular endothelium into the perivascular and interstitial spaces that serve as the main fluid conduits of the glymphatic system, however, it is unknown how different rates of ISF secretion affect glymphatic bulk flow. In chapter 2 of this thesis, plasma hyper- and hypotonicity were used to model low and high ISF secretion states, respectively. We found that low ISF formation led to decreased tissue volume and pressure, and as a result, glymphatic CSF influx to brain was enhanced. Conversely, high ISF formation increased brain water volume and tissue pressure, leading to impaired glymphatic influx, and surprisingly, improved interstitial solute clearance. Consequently, we conclude that modulation of ISF production holds promise for modifying brain diseases characterized by solute accumulation. In chapter 3, we performed a 3-dimensional biomechanical characterization of murine ‘Hit & Run’ TBI, demonstrating that linear head kinematics following dorsal impact TBI were inversely correlated with cerebral edema, while lateral TBI rotational kinematics were directly related to duration of loss of consciousness. As biomechanics were found to predict pathologic phenotype, we conclude that selection of a kinematically homogeneous sample will reduce variability in model pathology. Using this model, in chapter 4 we investigated the role of the glymphatic system in clearing endogenous molecules, specifically protein injury biomarkers, finding that clinical interventions that suppress glymphatic clearance reduced the appearance of these proteins in the blood. Finally, in chapter 5, we demonstrated that certain therapeutic approaches to TBI, here decompressive craniectomy, led to impaired glymphatic flow, and that the resulting glymphatic stagnation drove a neuroinflammatory response and neurologic deterioration. Consequently, efforts to preserve glymphatic flow, potentially through control of ISF secretion, may offer promise for future studies of TBI treatment

Paravascular microcirculation facilitates rapid lipid transport and astrocyte signaling in the brain

In the brain, a paravascular space exists between vascular cells and astroglial end-foot processes, creating a continuous sheath surrounding blood vessels. Using in vivo two-photon imaging we demonstrate that the paravascular circulation facilitates selective transport of small lipophilic molecules, rapid interstitial fluid movement and widespread glial calcium signaling. Depressurizing the paravascular system leads to unselective lipid diffusion, intracellular lipid accumulation and pathological signaling in astrocytes. As the central nervous system is devoid of lymphatic vessels, the paravascular space may serve as a lymphatic equivalent that represents a separate highway for the transport of lipids and signaling molecules

The pathophysiology underlying repetitive mild traumatic brain injury in a novel mouse model of chronic traumatic encephalopathy

BACKGROUND: An animal model of chronic traumatic encephalopathy (CTE) is essential for further understanding the pathophysiological link between repetitive head injury and the development of chronic neurodegenerative disease. We previously described a model of repetitive mild traumatic brain injury (mTBI) in mice that encapsulates the neurobehavioral spectrum characteristic of patients with CTE. We aimed to study the pathophysiological mechanisms underlying this animal model. METHODS: Our previously described model allows for controlled, closed head impacts to unanesthetized mice. Briefly, 12-week-old mice were divided into three groups: Control, single, and repetitive mTBI. Repetitive mTBI mice received six concussive impacts daily, for 7 days. Mice were then subsequently sacrificed for macro- and micro-histopathologic analysis at 7 days, 1 month, and 6 months after the last TBI received. Brain sections were immunostained for glial fibrillary acidic protein (GFAP) for astrocytes, CD68 for activated microglia, and AT8 for phosphorylated tau protein. RESULTS: Brains from single and repetitive mTBI mice lacked macroscopic tissue damage at all time-points. Single mTBI resulted in an acute rea ctive astrocytosis at 7 days and increased phospho-tau immunoreactivity that was present acutely and at 1 month, but was not persistent at 6 months. Repetitive mTBI resulted in a more marked neuroinflammatory response, with persistent and widespread astrogliosis and microglial activation, as well as significantly elevated phospho-tau immunoreactivity to 6-months. CONCLUSIONS: The neuropathological findings in this new model of repetitive mTBI resemble some of the histopathological hallmarks of CTE, including increased astrogliosis, microglial activation, and hyperphosphorylated tau protein accumulation

Aquaporin-4-dependent glymphatic solute transport in the rodent brain

The glymphatic system is a brain-wide clearance pathway; its impairment contributes to the accumulation of amyloid-β. Influx of cerebrospinal fluid (CSF) depends upon the expression and perivascular localization of the astroglial water channel aquaporin-4 (AQP4). Prompted by a recent failure to find an effect of Aqp4 knock-out (KO) on CSF and interstitial fluid (ISF) tracer transport, five groups re-examined the importance of AQP4 in glymphatic transport. We concur that CSF influx is higher in wild-type mice than in four different Aqp4 KO lines and in one line that lacks perivascular AQP4 (Snta1 KO). Meta-analysis of all studies demonstrated a significant decrease in tracer transport in KO mice and rats compared to controls. Meta-regression indicated that anesthesia, age, and tracer delivery explain the opposing results. We also report that intrastriatal injections suppress glymphatic function. This validates the role of AQP4 and shows that glymphatic studies must avoid the use of invasive procedures