PROJECT SUMMARY (Parent Grant)
Post-hemorrhagic hydrocephalus (PHH) is a leading cause of morbidity in premature infants. PHH is triggered
by germinal matrix intraventricular hemorrhage (IVH) that results in accumulation of cerebrospinal fluid (CSF)
in the brain, compression of surrounding brain tissue, and permanent neurological deficits. While PHH is
clearly caused by an altered balance of CSF production and removal, the mechanisms are poorly understood,
limiting our ability to guide rational therapies. Here, we propose to examine two processes that could be
manipulated therapeutically to alleviate PHH: (1) ion and fluid transport by the choroid plexus (ChP), and (2)
ventricular blood clearance by macrophages. In adults under normal physiological conditions, sheets of
specialized ChP epithelial cells secrete CSF via an incompletely understood set of membrane proteins
including NKCC1, a phosphorylation activated bi-directional Na-K-Cl cotransporter. Strikingly, we recently
discovered that NKCC1 participates in CSF removal rather than CSF secretion during early stages of brain
development. CSF-K+ levels are significantly higher in embryos than adults, likely explaining this opposite
direction of NKCC1 water transport. Experimental introduction of blood into the ventricles during development
appears to further elevate CSF-K levels, and to drive intracellular calcium activity in ChP epithelial cells,
expression of the immediate early gene c-fos, and increased expression/phosphorylation of NKCC1. Our
findings suggest a novel counter-regulatory response to IVH in premature infants: ChP absorption of CSF via
NKCC1, driven by K+. We will test this hypothesis by determining if NKCC1 activation either worsens or
mitigates hydrocephalus in our mouse IVH model (Aim 1; preliminary data suggests the latter). We also found
that following IVH, blood products linger in the developing ventricles and may account for the persistence of
PHH. The brain's ventricles and the apical surface of the ChP are home to specific macrophages known as
Kolmer cells. While Kolmer cells have been implicated as responders to brain hemorrhage, their scavenging
and other functions have remained elusive. Our data suggest that during early stages of brain development,
ventricular macrophages/Kolmer cells are activated and recruited to the site of blood leakage within the
ventricle (Aim 2A) and that these macrophages are necessary and sufficient to clear blood and/or inflammatory
signals from the ventricles (Aim 2B, C). Collectively, our data suggest that the ultimate severity of PHH
depends on a developmental stage-specific interplay between blood products, ion gradients (e.g. [K]), immune
and inflammatory reactions, and NKCC1 expression levels. An estimated 20% of infants that experience
intraventricular bleeds develop PHH. We suspect this is due to insufficient endogenous compensatory
responses. The ultimate goal of this proposal is to improve outcomes by laying the groundwork for
development of clinical treatments that boost endogenous removal of CSF and blood that drive the pathogenic
processes that lead to PHH. This proposal should also guide therapies for adult IVH and other conditions with
disrupted extracellular ionic homeostasis.
