Cannabinoid Receptors Modulate Neuronal Morphology and AnkyrinG Density at the Axon Initial Segment

Abstract

Neuronal polarization underlies the ability of neurons to integrate and transmit information. This process begins early in development with axon outgrowth, followed by dendritic growth and subsequent maturation. In between these two steps, the axon initial segment (AIS), a subcellular domain crucial for generating action potentials (APs) and maintaining the morphological and functional polarization, starts to develop. However, the cellular/molecular mechanisms and receptors involved in AIS initial development and maturation are mostly unknown. In this study, we have focused on the role of the type-1 cannabinoid receptor (CB1R), a highly abundant G-protein coupled receptor (GPCR) in the nervous system largely involved in different phases of neuronal development and differentiation. Although CB1R activity modulation has been related to changes in axons or dendrites, its possible role as a modulator of AIS development has not been yet explored. Here we analyzed the potential role of CB1R on neuronal morphology and AIS development using pharmacological and RNA interference approaches in cultured hippocampal neurons. CB1R inhibition, at a very early developmental stage, has no effect on axonal growth, yet CB1R activation can promote it. By contrast, subsequent dendritic growth is impaired by CB1R inhibition, which also reduces ankyrinG density at the AIS. Moreover, our data show a significant correlation between early dendritic growth and ankyrinG density. However, CB1R inhibition in later developmental stages after dendrites are formed only reduces ankyrinG accumulation at the AIS. In conclusion, our data suggest that neuronal CB1R basal activity plays a role in initial development of dendrites and indirectly in AIS proteins accumulation. Based on the lack of CB1R expression at the AIS, we hypothesize that CB1R mediated modulation of dendritic arbor size during early development indirectly determines the accumulation of ankyrinG and AIS development. Further studies will be necessary to determine which CB1R-dependent mechanisms can coordinate these two domains, and what may be the impact of these early developmental changes once neurons mature and are embedded in a functional brain network.

Type 2 Diabetes Leads to Axon Initial Segment Shortening in db/db Mice

“…Calcineurin, a calcium/calmodulin-dependent protein phosphatase, is responsible for both activity-dependent relocation (Evans et al, 2013 ) and shortening (Evans et al, 2015 ) of the AIS. Furthermore, AIS assembly, maintenance, and/or plasticity may also be regulated by cannabinoid receptors (Tapia et al, 2017 ), brain-derived neurotrophic factor and neurotrophin 3 (Guo et al, 2017 ), protein kinase CK2 (Bréchet et al, 2008 ; Hien et al, 2014 ; Xu and Cooper, 2015 ; Lezmy et al, 2017 ), Cdk5 (Trunova et al, 2011 ; Chand et al, 2015 ), microtubule cross-linking factor 1 (Satake et al, 2017 ), myosin II activity (Evans et al, 2017 ; Berger et al, 2018 ), or Rbfox splicing factors (Jacko et al, 2018 ). Future studies are required to identify the key mechanism of diabetes-related AIS changes.…”

Heterogeneity of the Axon Initial Segment in Interneurons and Pyramidal Cells of Rodent Visual Cortex

“…Thus, AIS location or length may be regulated differently in these cells compared to the BDNF-dependent FS PARV BCs. Interestingly, albeit not residing at the AIS, CB1R signaling influences the AIS during early differentiation in that inhibition of CB1R signaling decreases ankyrinG expression in dissociated hippocampal (mostly pyramidal) neurons, resulting in shorter AIS (Tapia et al, 2017 ). Presence and strength of such early influences may explain why in vivo and in vitro as well as in neurons from different species, the overall AIS length varies.…”

Chronic CB1 cannabinoid receptor antagonism persistently increases dendritic spine densities in brain regions important to zebra finch vocal learning and production in an antidepressant-sensitive manner

“…Less well-established are developmental effects following chronic antagonism, and thus our experiments are among the first to explore this area. Others have recently reported distinct effects of agonists vs. antagonists in cultured mouse hippocampal neurons, finding differential effects on morphology and distinct effects at different developmental stages (Tapia et al, 2017). A study with a design similar to ours, but employing rats and the CB1-selective antagonist AM-251, demonstrated both distinct effects following peri-adolescent vs. adult treatments and similar efficacies of agonist and antagonist on stress coping behaviors and expression levels of endocannabinoid signaling elements (Lee et al, 2015).…”