Antibody-directed extracellular proximity biotinylation reveals that Contactin-1 regulates axo-axonic innervation of axon initial segments

Tools to investigate the cell surface: Proximity as a central concept in glycoRNA biology

Proximity is a fundamental concept in chemistry and biology, referring to the convergence of molecules to facilitate new molecular interactions or reactions. Hybrid biopolymers like glycosylphosphatidylinositol (GPI)-anchored proteins, ubiquitinated proteins, glycosylated RNAs (glycoRNAs), and RNAylated proteins exemplify this by covalent bonding of moieties that are often orthogonally active. Hybrid molecules like glycoRNAs are localized to new physical spaces, generating new interfaces for biological functions. To fully investigate the compositional and spatial features of molecules like glycoRNAs, flexible genetic and chemical tools that encompass different encoding and targeting biopolymers are required. Here we discuss concepts of molecular proximity and explore newer proximity labeling technologies that facilitate applications in RNA biology, cell surface biology, and the interface therein with a particular focus on glycoRNA biology. We review the advantages and disadvantages of methods pertaining to cell surface RNA identification and provide insights into the vast opportunities for method development in this area.

TRIM46 is not required for axon specification or axon initial segment formation in vivo

Vertebrate nervous systems use the axon initial segment (AIS) to initiate action potentials and maintain neuronal polarity. The microtubule-associated protein tripartite motif containing 46 (TRIM46) was reported to regulate axon specification, AIS assembly, and neuronal polarity through the bundling of microtubules in the proximal axon. However, these claims are based on TRIM46 knockdown in cultured neurons. To investigate TRIM46 function in vivo , we examined TRIM46 knockout mice. Contrary to previous reports, we find that TRIM46 is dispensable for AIS formation and maintenance, and axon specification. TRIM46 knockout mice are viable, have normal behavior, and have normal brain structure. Thus, TRIM46 is not required for AIS formation, axon specification, or nervous system function. We also show TRIM46 enrichment in the first ∼100 μm of axon occurs independently of ankyrinG (AnkG), although AnkG is required to restrict TRIM46 only to the AIS. Our results suggest an unidentified protein may compensate for loss of TRIM46 in vivo and highlight the need for further investigation of the mechanisms by which the AIS and microtubules interact to shape neuronal structure and function.

SIGNIFICANCE STATEMENT

A healthy nervous system requires the polarization of neurons into structurally and functionally distinct compartments, which depends on both the axon initial segment (AIS) and the microtubule cytoskeleton. In contrast to previous reports, we show that the microtubule-associated protein TRIM46 is not required for axon specification or AIS formation in mice. Our results emphasize the need for further investigation of the mechanisms by which the AIS and microtubules interact to shape neuronal structure and function.

Synaptic proteomics decode novel molecular landscape in the brain

Synapses play a pivotal role in forming neural circuits, with critical implications for brain functions such as learning, memory, and emotions. Several advances in synaptic research have demonstrated the diversity of synaptic structure and function, which can form thousands of connections depending on the neuronal cell types. Moreover, synapses not only interconnect neurons but also establish connections with glial cells such as astrocytes, which play a key role in the architecture and function of neuronal circuits in the brain. Emerging evidence suggests that dysfunction of synaptic proteins contributes to a variety of neurological and psychiatric disorders. Therefore, it is crucial to determine the molecular networks within synapses in various neuronal cell types to gain a deeper understanding of how the nervous system regulates brain function. Recent advances in synaptic proteome approaches, such as fluorescence-activated synaptosome sorting (FASS) and proximity labeling, have allowed for a detailed and spatial analysis of many cell-type-specific synaptic molecules in vivo. In this brief review, we highlight these novel spatial proteomic approaches and discuss the regulation of synaptic formation and function in the brain. This knowledge of molecular networks provides new insight into the understanding of many neurological and psychiatric disorders.

Glia trigger endocytic clearance of axonal proteins to promote rodent myelination

Axons undergo striking changes in their content and distribution of cell adhesion molecules (CAMs) and ion channels during myelination that underlies the switch from continuous to saltatory conduction. These changes include the removal of a large cohort of uniformly distributed CAMs that mediate initial axon-Schwann cell interactions and their replacement by a subset of CAMs that mediate domain-specific interactions of myelinated fibers. Here, using rodent models, we examine the mechanisms and significance of this removal of axonal CAMs. We show that Schwann cells just prior to myelination locally activate clathrin-mediated endocytosis (CME) in axons, thereby driving clearance of a broad array of axonal CAMs. CAMs engineered to resist endocytosis are persistently expressed along the axon and delay both PNS and CNS myelination. Thus, glia non-autonomously activate CME in axons to downregulate axonal CAMs and presumptively axo-glial adhesion. This promotes the transition from ensheathment to myelination while simultaneously sculpting the formation of axonal domains.