Traumatized and inflamed--but resilient: glial aromatization and the avian brain

Abstract

Steroids like estrogens have potent effects on the vertebrate brain, and are provided to neural targets from peripheral and central sources. Estradiol synthesized within the vertebrate CNS modulates neural structure and function, including the pathways involved in neuroprotection, and perhaps, neural repair. Specifically, aromatase; the enzyme responsible for the conversion of testosterone to estradiol, is upregulated in the avian and mammalian brain following disruption of the neuropil by multiple forms of perturbation including mechanical injury, ischemia and excitotoxicity. This injury induced aromatase expression is somewhat unique in that it occurs in astroglia rather than neurons, and is stimulated in response to factors associated with brain damage. In this review, we focus on the induction, expression and consequences of glial aromatization in the songbird brain. We begin with a review of the anatomical consequences of glial estrogen provision followed by a discussion of the cellular mechanisms whereby glial aromatization may affect injury-induced neuroplasticity. We then present the current status of our understanding regarding the inductive role of inflammatory processes in the transcription and translation of astrocytic aromatase. We consider the functional aspects of glial aromatization before concluding with unanswered questions and suggestions for future studies. Birds have long informed us about fundamental questions in endocrinology, immunology, and neuroplasticity; and their unique anatomical and physiological characteristics continue to provide an excellent system in which to learn about brain trauma, inflammation, and neuroprotection.

Figure 1

Aromatase expression in the zebra finch brain. Constitutive neuronal (left) and induced glial (right) aromatase-positive cells in the zebra finch brain.

Figure 2

BMP2 expression following injury in the zebra finch brain. Injury induced (INJ) BMP2 expression in the adult zebra finch brain is non-neuronal. A) Western blots of telencephalic tissue homogenate from adult zebra finches reveal a ~50 kDa band that is consistent with the predicted molecular weight of zebra finch BMP2, when stained using an anti-BMP2 antibody (Abnova clone 1A11). B) INJ BMP2 positive cells (green) around the site of injury (dashed line) do not co-localize with the neuron specific proteins HuC/HuD (red) when examined at either 3 or 7 days after injury. However, INJ BMP2 is expressed in microglial-like cells in the adult zebra finch brain. C) BMP2 positive cells co-express the microglia/macrophage specific protein macrosialin (CD68). BMP2 positive cells (green) in other parts of the zebra finch brain, including the diencephalon (Dienc) are not co-labeled by the CD68 antibody (red). Constitutive BMP2 expression is primarily neuronal, as typified by immunostaining in the Entopallium (Ento) where BMP2 co-express the neuronal proteins HuC/HuD (stained using the mouse anti HuC/HuD antibody, clone 16A11, from Invitrogen). Macrosialin was immunostained using the rat anti-CD68 antibody (clone FA- 11) from AbD Serotec.

Figure 3

Antagonism of BMP signaling around cerebral injury in the adult zebra finch brain results in increased lesion volume. (A) Affi-Gel Blue agarose beads (Bio-rad) were soaked in the BMP antagonist noggin (0.5 mg/mL of 10% BSA) and injected into the cerebrum of adult zebra finches. (B) Contralateral hemispheres were injected with Afii-Gel Blue beads that were soaked in 10% bovine serum albumin (BSA) only. Rabbit anti-phospho Smad 1/5/8 staining (Millipore AB3848) demonstrates an area devoid of BMP signaling activity around the primary insult (A) while no such absence of pSmad 1/5/8 staining was seen in BSA treated controls (B). Treatment of injured hemispheres with noggin resulted in greater lesion volume than in hemispheres treated with BSA only when examined 72 hrs. after injury (C). *p = 0.035, Mann-Whitney test.

Figure 4

Neuroinflammation induces glial aromatase in the uninjured brain. A) Photomicrographs illustrating the induction of aromatase positive glial cells following with PHA induced neuroinflammatory treatment or saline. B) Photomicrograph of PHA-induced cells at a higher magnification. C) Aromatase mRNA expression following PHA treatment, analyzed using qPCR. D) Similar to aromatase positive cells following injury, PHA induced glial cells (red) also co-localize with Vimentin (green); arrow heads denote areas of co-localization. Images modified from Duncan & Saldanha, 2011. * denotes a significant difference between treatment.