Unique biology of gliomas: challenges and opportunities

Abstract

Gliomas are terrifying primary brain tumors for which patient outlook remains bleak. Recent research provides novel insights into the unique biology of gliomas. For example, these tumors exhibit an unexpected pluripotency that enables them to grow their own vasculature. They have an unusual ability to navigate tortuous extracellular pathways as they invade, and they use neurotransmitters to inflict damage and create room for growth. Here, we review studies that illustrate the importance of considering interactions of gliomas with their native brain environment. Such studies suggest that gliomas constitute a neurodegenerative disease caused by the malignant growth of brain support cells. The chosen examples illustrate how targeted research into the biology of gliomas is yielding new and much needed therapeutic approaches to this challenging nervous system disease.

Figure 1

Glioma cells establish a clear relationship and interdependence with the cerebral vasculature. (a) Gliomas comprise a heterogeneous population of tumor cells, including glioma stem cells, which are capable of propagating the tumor itself and differentiating into glioma endothelial progenitor cells [–32]. These endothelial progenitor cells differentiate into glioma-derived endothelial cells (purple cells) and facilitate tumor-mediated neovascularization. (b) Confocal microscopy images of vasculature demonstrating the differentiation of human glioma cells into vascular endothelial cells when implanted into a mouse [31]. Vasculature identified with cluster of differentiation 31 (CD31) and lectin (green). Human origin confirmed via human-specific endothelial markers, neural cell adhesion molecule (NCAM) and CD105 (red). DAPI staining (blue) was used to indicate cell nuclei. Scale bar = 10 µm. (c) Glioma cells preferentially migrate and invade along the abluminal surface of the vasculature [36]. In vivo demonstration of the intimate association of human glioma cells (green) with the vasculature (red) in a xenograft model as viewed through a cranial window in a live mouse. 600X magnification. Scale bar = 50 µm. (d) Schematic of bradykinin receptor (B2R) signaling that mediates glioma attraction to the vasculature. Glioma cells express the B2R, which binds vascular endothelial cell-released bradykinin (BK) [37]. The initiation of this signaling cascade facilitates glioma association with blood vessels. Reproduced, with permission, from [31] (b) and [36] (c).

Figure 2

Glioma cells extrude high concentrations of glutamate (Glu) via the system X_c⁻ transporter (SXC), supporting tumor growth and expansion [54,60,62]. (a) Glu is released from glioma cells in exchange for cystine, which is converted into the antioxidant glutathione, protecting glioma cells from free radical damage. Simultaneously, overwhelming amounts of extracellular Glu cannot be sufficiently removed by astrocytes through normal uptake mechanisms involving the excitatory amino acid transporters excitatory amino-acid transporter 1 (EAAT1) and EAAT2. This leads to prolonged neuronal exposure to Glu, resulting in neuronal hyperexcitability and death [51,52,56,60]. (b) Glu release from gliomas by the system X_c⁻ transporter (SXC) incites epileptic activity exhibited by an electroencephalography (EEG) recording from a tumor-implanted mouse [67]. Reproduced, with permission, from [67] (EEG recording) and [86] (MRI of xenograft tumor).

Figure 3

The movement of ions and water facilitates the cellular volume changes needed for glioma cell proliferation, invasion, and migration [,–79]. (a) Glioma cells undergo significant shape and volume changes during the cell cycle, including a dramatic decrease in volume just before M phase. Cell volumes were determined after three-dimensional rendering of up to 400 individual optical sections, (400 µm z-spacing), as shown in 1–5 taken by confocal imaging [79]. (b) Volume of glioma cells at M phase, the last time point measured before M phase (Pre M Phase), and the maximum volume (Peak) measured for each glioma cell [79]. (c) Schematic of the ion channels and co-transporters used to establish ionic gradients to promote water flux across the cell membrane allowing for volume changes needed. Abbreviations: ATPase, sodium-potassium ATPase; ClC-3, voltage-gated chloride channel 3; K_Ca²⁺, calcium-activated potassium channels; NKCC1, sodium-potassium-chloride co-transporter isoform-1. Reproduced, with permission, from [79] (a,b).