The 20–60-μm axon initial segment (AIS) is proximally located at the interface between the axon and cell body. AIS has characteristic molecular and structural properties regulated by the crucial protein, ankyrin-G. The AIS contains a high density of Na+ channels relative to the cell body, which allows low thresholds for initiation of action potential (AP). Molecular and physiological studies have shown that the AIS is also a key domain for the control of neuronal excitability by homeostatic mechanisms. The AIS has high plasticity in normal developmental processes and pathological activities such as injury, neurodegeneration, and neurodevelopmental disorders (NDDs). In the first half of this review, we provide an overview of the molecular, structural, and ion-channel characteristics of AIS, AIS regulation through axo-axonic synapses, and axo-glial interactions. In the second half, to understand the relationship between NDDs and AIS, we discuss the activity-dependent plasticity of AIS, the human mutation of AIS regulatory genes, and the pathophysiological role of the abnormal AIS in the NDD model animals and patients. We propose that AIS may provide a potentially valuable structural biomarker in response to abnormal network activity in vivo as well as a new treatment concept at the neural circuit level.

Patients with neurodegenerative diseases (NDDs) are rapidly increasing all over the world. Alzheimer’s disease (AD), Parkinson’s disease, Huntington’s disease (HD), frontotemporal lobar degeneration (FTLD) and many other diseases are classified as NDDs, however, no therapeutic drugs have been developed against NDDs yet. As common phenomenon, protein oligomers and aggregates (inclusion bodies) are formed in neurons, and they trigger neuronal cell death and/or axonal degeneration. We have investigated the mechanisms of NDDs, and discovered several chemicals and peptides have potential to cure NDDs. Here, we show phorbol 12-myristate 13-acetate (PMA) significantly inhibits aggregate formation and promotes neurite regeneration in human neuronal cell culture model of NDDs, FTLD and AD. We expressed R406W mutant tau protein found in FTLD patients in human neuroblastoma SH-SY5Y cells as FTLD cell culture model. These FTLD neuronal cells showed intracellular aggregate formation and impaired neurite growth. Addition of PMA to culture medium 60% suppressed aggregate formation. PMA also enhanced neurite growth to the normal level at nanomolar concentration. We cultured SH-SY5Y cells in the medium containing -amyloid (1-42) as AD cell culture model. In these cells, neurite growth was limited to less than 50% of controls by the treatment of -amyloid (1-42). Surprisingly, addition of PMA increased the number of cells with >10-fold neurite extension to seven times even in -amyloid (1-42) contained medium. Also, PMA increased the expression of several genes and proteins important for cell survival in both cell culture models. These results suggest PMA has unexpected potential for the drug development against NDDs.

The Golgi apparatus (GA) dysfunctions in Parkinson’s Disease (PD), neurodevelopmental disorders (NDDs), cancer, and organelle structural biology (OSB) can provide insights into therapeutic targets, gene therapy, and drug design. Primary defects and fragmentation within the GA are implicated in a wide range of neurodegenerative diseases. GA defects typically result in mislocation of proteins, accumulation of undegraded proteins, and impaired glycosylation of proteins. Inhibition of vesicular trafficking by α-synuclein (aSyn) may affect the dopamine-producing neurons and neuromodulators. GA regulates apoptosis during pathological mechanisms of neurological diseases and could provide new avenues in treatments through translation research. PD patients bearing the hereditary E46K disease mutation manifest the clinical picture of parkinsonism. How do we provide high resolution nanoimages of the GA during disease to capture dysfunction? Could we visualize the aSyn traffic jam between vesicles in the organelles ER and GA? OSB is emerging as a field as more technology advances and is more accessible. Structural studies of the GA will advance the field of neurological disease forward with an in depth understanding of dysfunction, fragmentation, and defects. Discoveries of the GA in PD, NDDs, and cancer would break new ground and provide translational medicine data of these diseases. Future research could be visualizing high angle annular dark field-STEM (HAADF-STEM) tomograms, cryogenic electron tomography (cryo-ET), multiplex correlative light and electron microscopy (cryo-CLEM), nanobody-assisted tissue immunostaining for volumetric EM (NATIVE) and using soft X-ray tomography (SXT) and computational reconstruction of the GA.