You need dendritic branching to get around white and grey matter dead areas. So ask your doctor what protocols they have set up for this.
A two-step actin polymerization mechanism drives dendrite branching
Neural Development volume 16, Article number: 3 (2021)
Abstract
Background
Dendrite morphogenesis plays an essential role in establishing the connectivity and receptive fields of neurons during the development of the nervous system. To generate the diverse morphologies of branched dendrites, neurons use external cues and cell surface receptors to coordinate intracellular cytoskeletal organization; however, the molecular mechanisms of how this signaling forms branched dendrites are not fully understood.
Methods
We performed in vivo time-lapse imaging of the PVD neuron in C. elegans in several mutants of actin regulatory proteins, such as the WAVE Regulatory Complex (WRC) and UNC-34 (homolog of Enabled/Vasodilator-stimulated phosphoprotein (Ena/VASP)). We examined the direct interaction between the WRC and UNC-34 and analyzed the localization of UNC-34 in vivo using transgenic worms expressing UNC-34 fused to GFP.
Results
We identify a stereotyped sequence of morphological events during dendrite outgrowth in the PVD neuron in C. elegans. Specifically, local increases in width (“swellings”) give rise to filopodia to facilitate a “rapid growth and pause” mode of growth. In unc-34 mutants, filopodia fail to form but swellings are intact. In WRC mutants, dendrite growth is largely absent, resulting from a lack of both swelling and filopodia formation. We also found that UNC-34 can directly bind to the WRC. Disrupting this binding by deleting the UNC-34 EVH1 domain prevented UNC-34 from localizing to swellings and dendrite tips, resulting in a stunted dendritic arbor and reduced filopodia outgrowth.
Conclusions
We propose that regulators of branched and linear F-actin cooperate to establish dendritic branches. By combining our work with existing literature, we propose that the dendrite guidance receptor DMA-1 recruits the WRC, which polymerizes branched F-actin to generate “swellings” on a mother dendrite. Then, WRC recruits the actin elongation factor UNC-34/Ena/VASP to initiate growth of a new dendritic branch from the swelling, with the help of the actin-binding protein UNC-115/abLIM. Extension of existing dendrites also proceeds via swelling formation at the dendrite tip followed by UNC-34-mediated outgrowth. Following dendrite initiation and extension, the stabilization of branches by guidance receptors further recruits WRC, resulting in an iterative process to build a complex dendritic arbor.
Background
Neurite outgrowth and arborization are essential for the establishment and function of neural circuits. In particular, the morphology of dendrites is key for neuronal signal transmission and integration. Dendrite morphogenesis is often guided by extracellular cues recognized through a number of cell surface receptors, such as Semaphorins, DSCAMs, and protocadherins, which in turn instruct activities of various intracellular molecules leading to cytoskeletal reorganization [1, 2]. Similarly, activity-dependent dendritic arbor development also requires cytoskeletal signaling, including through Rho GTPase activity [3] or microtubule stabilization [4].
One major signaling axis for neurite outgrowth is the activation and membrane recruitment of the actin nucleation promotion factor, the WAVE Regulatory Complex (WRC), a pentameric complex comprising Sra1/Cyfip1, Nap1/Hem-2, Abi, HSPC300, and WAVE/Scar [5]. After being activated and recruited by diverse membrane receptors and the Rho family GTPase Rac1, the WRC can then stimulate the Arp2/3 complex to produce branched actin networks [6]. In addition, a distinct actin elongation factor, Ena/VASP, can be recruited downstream of the Netrin receptor, DCC, [7,8,9] to promote polarized filopodia formation during axon guidance. Ena/VASP can also be recruited by the Slit receptor, Robo, to mediate axon repulsion [10, 11]. Interestingly, these distinct actin regulators can act synergistically in several morphological processes. For example, the direct interaction between the WRC and Ena/VASP was shown to play an important role in photoreceptor axon targeting, oogenesis, and macrophage migration in Drosophila, lamellipodia formation during ventral closure of C. elegans embryogenesis, and collective axon extension in mice [12,13,14]. It is, however, largely unknown how the activity of distinct actin regulators is coordinated in complex morphological processes.
One hypothesis is that complex processes like neuronal morphogenesis integrate structurally distinct actin networks built by different actin regulators. For example, in dendritic spines of hippocampal pyramidal neurons, patches of loosely bundled linear actin filaments constitute the dendritic spine neck, whereas branched actin networks drive the expansion of spine heads [15]. Furthermore, Abl tyrosine kinase has been shown to downregulate Ena activity while in parallel activating Rac/WAVE signaling, thus coordinating the balance of linear actin bundles and branched actin networks to regulate Drosophila axon patterning [16]. Finally, branched actin networks generated via the Arp2/3 complex were found to provide sites for initiating new dendrite branches in Drosophila larval sensory da neurons [17]. Therefore, it is likely neurons control morphological changes by dynamically orchestrating the formation of distinct types of actin networks in a coherent manner.
In this work, we provide evidence in support of this hypothesis showing that in the C. elegans PVD neuron, the development of dendritic arbors requires two distinct, but cooperative, steps of actin assembly that involve the WRC, UNC-34 (Ena/VASP), and the actin-binding protein UNC-115 (abLIM). As a recently developed model system for studying dendritic morphogenesis, the PVD neuron elaborates complex dendritic arbors by extending a primary dendrite along the length of the worm, followed by orthogonal secondary, tertiary, and quaternary dendrites, which together create stereotyped “menorah”-like structures [18]. A multipartite ligand-receptor complex, consisting of the extracellular ligands SAX-7, MNR-1, and LECT-2, along with the PVD guidance receptor DMA-1 and its partner HPO-30, directs the growth of PVD dendrites along the epidermis and body wall muscles [19,20,21,22,23]. This ligand-receptor complex recruits actin regulators, including the Rac GEF TIAM-1 and the WRC, through the cytosolic domains of DMA-1 and HPO-30, respectively, which in turn produce F-actin in growing dendrite tips to drive dendrite branching [23,24,25]. However, the mechanisms of F-actin recruitment during distinct morphological steps during dendrite development, such as initiation of new branches and branch elongation, remain unclear.
Here, we show that new dendrite branch points are established first as local “swellings” along dendrites, from which filopodia sprout to enable rapid and efficient outgrowth of dendritic branches. The formation of swellings is mediated by the WRC, while the subsequent extension of filopodial branches is mediated by UNC-34/Ena/VASP and UNC-115/abLIM. Our data suggest that a direct interaction between the WRC and UNC-34 provides the mechanism by which two distinct actin assembling processes are coupled to drive dendrite arborization.
No comments:
Post a Comment