Morphogenesis and regulation of Bergmann glial processes during Purkinje cell dendritic spine ensheathment and synaptogenesis

Abstract

Astrocytes have an important role in synaptic formation and function but how astrocytic processes become associated with synaptic structures during development is not well understood. Here we analyzed the pattern of growth of the processes extending off the main Bergmann glial (BG) shafts during synaptogenesis in the cerebellum. We found that during this period BG process outgrowth was correlated with increased ensheathment of dendritic spines. In addition, two-photon time-lapse imaging revealed that BG processes were highly dynamic, and processes became more stable as the period of spine ensheathment progressed. While process motility was dependent on actin polymerization, activity of cytoskeletal regulators Rac1 and RhoG did not play a role in glial process dynamics or density, but was critical for maintaining process length. We extended this finding to probe the relationship between process morphology and ensheathment, finding that shortened processes result in decreased coverage of the spine. Furthermore, we found that areas in which BG expressed dn-Rac1, and therefore had a lower level of synaptic ensheathment, showed an overall increase in synapse number. These analyses reveal how BG processes grow to surround synaptic structures, elucidate the importance of BG process structure for proper development of synaptic ensheathment, and reveal a role for ensheathment in synapse formation.

Morphological plasticity of astroglia: Understanding synaptic microenvironment

“…Very fine, and apparently highly mobile, astrocytic processes have been reported in organotypic brain slices using a membrane‐bound GFP (Benediktsson et al, 2005). Furthermore, co‐labeling of dendritic spines and PAPs in hippocampal (Halassa et al, 2007a; Perez‐Alvarez et al, 2014; Verbich et al, 2012) and cerebellar slices (Lippman Bell et al, 2010; Lippman et al, 2008) as well as in vivo (Bernardinelli et al, 2014b; Perez‐Alvarez et al, 2014) suggested that PAPs are highly dynamic structures that engage and disengage with dendritic spines, also responding to the induction of synaptic plasticity (Bernardinelli et al, 2014b; Perez‐Alvarez et al, 2014). …”

“…In cultures (Safavi‐Abbasi et al, 2001) as well as slices (Lippman et al, 2008; Nestor et al, 2007; Nishida and Okabe, 2007) astrocytic processes can adapt quickly to changes in their environment through a mechanism involving the GTPase Rac‐1. Moreover, expression of a dominant negative form of the actin‐binding protein profilin‐1 inhibited filopodia formation and motility (Molotkov et al, 2013).…”

“…Furthermore, blocking Rac‐1 and interfering with PAP motility leads to enhanced number of dendritic filopodia (Nishida and Okabe, 2007). Similarly, in the cerebellum Bergmann glia coverage is required for synapse stabilization and regulation (Lippman Bell et al, 2010; Lippman et al, 2008). …”

Glia delimit shape changes of sensory neuron receptive endings in C. elegans

“…Indeed, all dendritic spines examined by EM in the cerebellum are tightly ensheathed by Bergmann glia (Spacek, 1985), and most spines examined by EM in the hippocampus are ensheathed by astrocytes (Ventura and Harris, 1999). Furthermore, Bergmann glia have been shown to affect neuronal receptive ending shapes in the cerebellum (Lippman et al, 2008), and astrocytic glia affect dendritic spine morphology of hippocampal neurons via ephrin-A3/EphA4 signaling (Murai et al, 2003). At least two possible mechanisms by which the glial compartment can delimit receptive-ending shape are possible.…”

“…The proximity of receptive endings and glial processes might reflect more dynamic interactions between these cellular protrusions. For example, in the cerebellum, Bergmann glia extend and retract processes in concert with the emergence and regression of Purkinje cell dendritic spines (Lippman et al, 2008). Similarly, during lactation in female rats the processes of hypothalamic astrocytic glia retract, and this is accompanied by synaptic remodeling of the glia-ensheathed supraoptic nucleus (SON) neurons (Theodosis and Poulain, 1993).…”