Evidence that the influence of ganglion cell axons on astrocyte morphology is mediated by action spike activity during development

Astrocytes: inexplicable cells in neurodegeneration

Astrocytes are the most explored non-neuronal cells in the brain under neurophysiological and neurodegenerative conditions. Extensive research has been done to understand their specific role during neuropathological conditions but still the existing findings could not conclude their mechanism of action and their specific role in neurodegenerative conditions. This review discusses their physiological and pathological roles, their activation, morphological alterations and their probable use in search of new therapeutic targets for the treatment of neurodegenerative diseases.

Astrocytes and Müller Cell Alterations During Retinal Degeneration in a Transgenic Rat Model of Retinitis Pigmentosa

Purpose: Retinitis pigmentosa includes a group of progressive retinal degenerative diseases that affect the structure and function of photoreceptors. Secondarily to the loss of photoreceptors, there is a reduction in retinal vascularization, which seems to influence the cellular degenerative process. Retinal macroglial cells, astrocytes, and Müller cells provide support for retinal neurons and are fundamental for maintaining normal retinal function. The aim of this study was to investigate the evolution of macroglial changes during retinal degeneration in P23H rats.

Methods: Homozygous P23H line-3 rats aged from P18 to 18 months were used to study the evolution of the disease, and SD rats were used as controls. Immunolabeling with antibodies against GFAP, vimentin, and transducin were used to visualize macroglial cells and cone photoreceptors.

Results: In P23H rats, increased GFAP labeling in Müller cells was observed as an early indicator of retinal gliosis. At 4 and 12 months of age, the apical processes of Müller cells in P23H rats clustered in firework-like structures, which were associated with ring-like shaped areas of cone degeneration in the outer nuclear layer. These structures were not observed at 16 months of age. The number of astrocytes was higher in P23H rats than in the SD matched controls at 4 and 12 months of age, supporting the idea of astrocyte proliferation. As the disease progressed, astrocytes exhibited a deteriorated morphology and marked hypertrophy. The increase in the complexity of the astrocytic processes correlated with greater connexin 43 expression and higher density of connexin 43 immunoreactive puncta within the ganglion cell layer (GCL) of P23H vs. SD rat retinas.

Conclusions: In the P23H rat model of retinitis pigmentosa, the loss of photoreceptors triggers major changes in the number and morphology of glial cells affecting the inner retina.

mGluR6 deletion renders the TRPM1 channel in retina inactive

In darkness, glutamate released from photoreceptors activates the metabotropic glutamate receptor 6 (mGluR6) on retinal ON bipolar cells. This activates the G protein G(o), which then closes transient receptor potential melastatin 1 (TRPM1) channels, leading to cells' hyperpolarization. It has been generally assumed that deleting mGluR6 would render the cascade inactive and the ON bipolar cells constitutively depolarized. Here we show that the rod bipolar cells in mGluR6-null mice were hyperpolarized. The slope conductance of the current-voltage curves and the current noise were smaller than in wild type. Furthermore, while in wild-type rod bipolar cells, TRPM1 could be activated by local application of capsaicin; in null cells, it did not. These results suggest that the TRPM1 channel in mGluR6-null rod bipolar cells is inactive. To explore the reason for this lack of activity, we tested if mGluR6 deletion affected expression of cascade components. Immunostaining for G protein subunit candidates Gα(o), Gβ(3), and Gγ(13) showed no significant changes in their expression or distribution. Immunostaining for TRPM1 in the dendritic tips was greatly reduced, but the channel was still present in the soma and primary dendrites of mGluR6-null bipolar cells, where a certain fraction of TRPM1 appears to localize to the plasma membrane. Consequently, the lack of TRPM1 activity in the null retina is unlikely to be due to failure of the channels to localize to the plasma membrane. We speculate that, to be constitutively active, TRPM1 channels in ON bipolar cells have to be in a complex, or perhaps require an unidentified factor.

Activity-dependent neuron-glial signaling by ATP and leukemia-inhibitory factor promotes hippocampal glial cell development

Activity-dependent signaling between neurons and astrocytes contributes to experience-dependent plasticity and development of the nervous system. However, mechanisms responsible for neuron-glial interactions and the releasable factors that underlie these processes are not well understood. The pro-inflammatory cytokine, leukemia-inhibitory factor (LIF), is transiently expressed postnatally by glial cells in the hippocampus and rapidly up-regulated by enhanced neural activity following seizures. To test the hypothesis that spontaneous neural activity regulates glial development in hippocampus via LIF signaling, we blocked spontaneous activity with the sodium channel blocker tetrodotoxin (TTX) in mixed hippocampal cell cultures in combination with blockers of LIF and purinergic signaling. TTX decreased the number of GFAP-expressing astrocytes in hippocampal cell culture. Furthermore, blocking purinergic signaling by P2Y receptors contributed to reduced numbers of astrocytes. Blocking activity or purinergic signaling in the presence of function-blocking antibodies to LIF did not further decrease the number of astrocytes. Moreover, hippocampal cell cultures prepared from LIF -/- mice had reduced numbers of astrocytes and activity-dependent neuron-glial signaling promoting differentiation of astrocytes was absent. The results show that endogenous LIF is required for normal development of hippocampal astrocytes, and this process is regulated by spontaneous neural impulse activity through the release of ATP.

The differential response of astrocytes within the vestibular and cochlear nuclei following unilateral labyrinthectomy or vestibular afferent activity blockade by transtympanic tetrodotoxin injection in the rat

In this study, we investigated whether changes in the vestibular neuronal activity per se influence the pattern of astrocytes morphology, glial fibrillary acidic protein (GFAP) expression and ultimately their activation within the vestibular nuclei after unilateral transtympanic tetrodotoxin (TTX) injections and after unilateral inner ear lesion. The rationale was that, theoretically the noninvasive pharmacological functional blockade of peripheral vestibular inputs with TTX, allowed us to dissociate the signals exclusively related to the shutdown of the resting activity of the first-order vestibular neurons and from neuronal signals associated with trans-ganglionic changes in first order vestibular neurons induced by unilateral labyrinthectomy (UL). Since the cochlea was removed during the surgical procedure, we also studied the astrocytic reaction within the deafferented cochlear nuclei. No significant changes in the distribution or relative levels of GFAP mRNA expression, relative levels of GFAP protein or immunoreactivity for GFAP were found in the ipsilateral vestibular nuclei at any post-TTX injection times studied. In addition, no sign of microglia activation was observed. In contrast, a robust increase of the distribution and relative levels of GFAP mRNA expression, protein levels and immunoreactivity was observed in the deafferented vestibular and cochlear nuclei beginning at 1 day after inner ear lesion. GFAP mRNA expression and immunoreactivity in the cochlear nucleus was qualitatively stronger than in the ipsilateral vestibular nuclei. The results suggest that astrocyte activation in the vestibular nuclei is not related to drastic changes of vestibular nuclei neuronal activity per se. Early trans-ganglionic changes due to vestibular nerve dendrites lesion provoked by the mechanical destruction of vestibular receptors, most probably induced the glial reaction. Its functional role in the vestibular compensation process remains to be elucidated.

Neurons set the tone of gap junctional communication in astrocytic networks

A number of studies have contributed to demonstrate that neurons and astrocytes tightly and actively interact. Indeed, the presence of astrocytes in neuronal cultures increases the number of synapses and their efficiency, and thanks to enzymatic and uptake processes, astrocytes play a role in neuroprotection. A typical feature of astrocytes is that they establish cell-cell communication in vitro, as well as in situ, through intercellular channels forming specialized membrane areas defined as gap junctions. These channels are composed of junctional proteins termed connexins (Cxs): in astrocytes connexin 43 (Cx43) and 30 (Cx30) have been shown to prevail. Several recent works indicate that gap junctional communication (GJC) and/or connexin expression in astrocytes are controlled by neurons. Altogether, these observations lead to the concept that neuronal and astrocytic networks interact through mutual setting of their respective mode of communication and that astrocyte gap junctions represent a target in neuroglial interaction.

Cross-talk between neurons and glia: highlights on soluble factors

The development of the nervous system is guided by a balanced action between intrinsic factors represented by the genetic program and epigenetic factors characterized by cell-cell interactions which neural cells might perform throughout nervous system morphogenesis. Highly relevant among them are neuron-glia interactions. Several soluble factors secreted by either glial or neuronal cells have been implicated in the mutual influence these cells exert on each other. In this review, we will focus our attention on recent advances in the understanding of the role of glial and neuronal trophic factors in nervous system development. We will argue that the functional architecture of the brain depends on an intimate neuron-glia partnership.