UVM Theses and Dissertations
Format:
Print
Author:
Jebbett, Nathan J.
Dept./Program:
Neurology
Year:
2014
Degree:
PhD
Abstract:
Methylmercury (MeHg) is a developmental neurotoxin that concentrates in aquatic food chains and poses its highest risk to the offspring of mothers that consume contaminated seafood during pregnancy. Low doses can lead to a spectrum of neurodevelopmental impairments in domains such as IQ, perception, learning, attention, memory and motor coordination. The precise nature of impairment is likely determined by an interaction between dose and timing of exposure in relation to the ongoing formation of the nervous system. However, the specific pathological mechanisms responsible for damage at lower doses are not well known and likely distinct from the mechanisms determined in experiments examining neurodegeneration induced by higher doses of MeHg.
A growing body of evidence illustrates that neural progenitor cells are sensitive to MeHg; these self-renewing multipotent cells generate subtypes of amplifying cells and ultimately give rise to neurons, astrocytes, and oligodendrocytes. Although the predominant focus of studies has been on neuronal lineages, the rapid expansion of glial progenitor cells is responsible for much of the growth in brain volume that starts in the early third trimester for humans and near birth in rodents, coinciding with a neurodyvelopmental window of greatest sensitivity to MeHg. We hypothesized that signals involved in astrocyte and oligodendrocyte differentiation could be targets of MeHg at concentrations relevant to envIronmental exposures.
To test this, we modeled astrocyte lineage differentiation in neural progenitor cells derived from postnatal day (PND) 2 mouse cortex, focusing on the JAKJSTAT signaling pathway, a central regulator of gliogenesis involved in promoting astrocyte differentiation and oligodendrocyte survival. Low amounts of MeHg (30-300 nM) increased the proportion of astrocyte-like cells generated in vitro from neural precursors differentiating in response to ciliary neurotrophic factor, enhanced STAT3 phosphorylation and increased transcription of STAT3-responsive genes. The mechanism for these effects did not involve increased oxidative stress, as no changes in superoxide anion concentrations nor oxidative-response genes were noted.
To further characterize effects of MeHg on gliogenesis in vivo, we administered a single injection of 20 mg/kg MeHg with bromodeoxyuridine (Br4U) in postnatal day (PND2) 2 and PND6 mouse pups and analyzed markers of glial lineage in a sample region of the cortex 4 days after injection. MeHg treatment on PND2 increased the amount of GFAP immunoreactivity in the underlying white matter, and caused more mature S100[Beta] astrocytes to label with BrdU, suggesting enhanced astrocyte cell division. These changes were not observed in specimens from in animals injected on PND6, where increased BrdU⁺ cells and platelet-derived growth factor receptor alpha (PDGFR[Alpha]⁺) oligodendrocyte progenitors were noted.
Our results support the concept that low doses of MeHg affect glial development, and that the precise nature of this effect depends on the timing of exposure during development.
A growing body of evidence illustrates that neural progenitor cells are sensitive to MeHg; these self-renewing multipotent cells generate subtypes of amplifying cells and ultimately give rise to neurons, astrocytes, and oligodendrocytes. Although the predominant focus of studies has been on neuronal lineages, the rapid expansion of glial progenitor cells is responsible for much of the growth in brain volume that starts in the early third trimester for humans and near birth in rodents, coinciding with a neurodyvelopmental window of greatest sensitivity to MeHg. We hypothesized that signals involved in astrocyte and oligodendrocyte differentiation could be targets of MeHg at concentrations relevant to envIronmental exposures.
To test this, we modeled astrocyte lineage differentiation in neural progenitor cells derived from postnatal day (PND) 2 mouse cortex, focusing on the JAKJSTAT signaling pathway, a central regulator of gliogenesis involved in promoting astrocyte differentiation and oligodendrocyte survival. Low amounts of MeHg (30-300 nM) increased the proportion of astrocyte-like cells generated in vitro from neural precursors differentiating in response to ciliary neurotrophic factor, enhanced STAT3 phosphorylation and increased transcription of STAT3-responsive genes. The mechanism for these effects did not involve increased oxidative stress, as no changes in superoxide anion concentrations nor oxidative-response genes were noted.
To further characterize effects of MeHg on gliogenesis in vivo, we administered a single injection of 20 mg/kg MeHg with bromodeoxyuridine (Br4U) in postnatal day (PND2) 2 and PND6 mouse pups and analyzed markers of glial lineage in a sample region of the cortex 4 days after injection. MeHg treatment on PND2 increased the amount of GFAP immunoreactivity in the underlying white matter, and caused more mature S100[Beta] astrocytes to label with BrdU, suggesting enhanced astrocyte cell division. These changes were not observed in specimens from in animals injected on PND6, where increased BrdU⁺ cells and platelet-derived growth factor receptor alpha (PDGFR[Alpha]⁺) oligodendrocyte progenitors were noted.
Our results support the concept that low doses of MeHg affect glial development, and that the precise nature of this effect depends on the timing of exposure during development.