Morphological correlates of pyramidal cell axonal myelination in mouse and human neocortex

The axons of neocortical pyramidal neurons are frequently myelinated. Heterogeneity in the topography of axonal myelination in the cerebral cortex has been attributed to a combination of electrophysiological activity, axonal morphology, and neuronal-glial interactions. Previously, we showed that axonal segment length and caliber are critical local determinants of fast-spiking interneuron myelination. However, the factors that determine the myelination of individual axonal segments along neocortical pyramidal neurons remain largely unexplored. Here, we used structured illumination microscopy to examine the extent to which axonal morphology is predictive of the topography of myelination along neocortical pyramidal neurons. We identified critical thresholds for axonal caliber and interbranch distance that are necessary, but not sufficient, for myelination of pyramidal cell axons in mouse primary somatosensory cortex (S1). Specifically, we found that pyramidal neuron axonal segments with a caliber < 0.24 μm or interbranch distance < 18.10 μm are rarely myelinated. Moreover, we further confirmed that these findings in mice are similar for human neocortical pyramidal cell myelination (caliber < 0.25 μm, interbranch distance < 19.00 μm), suggesting that this mechanism is evolutionarily conserved. Taken together, our findings suggest that axonal morphology is a critical correlate of the topography and cell-type specificity of neocortical myelination.

Reconstruction of 1,000 Projection Neurons Reveals New Cell Types and Organization of Long-Range Connectivity in the Mouse Brain

Neuronal cell types are the nodes of neural circuits that determine the flow of information within the brain. Neuronal morphology, especially the shape of the axonal arbor, provides an essential descriptor of cell type and reveals how individual neurons route their output across the brain. Despite the importance of morphology, few projection neurons in the mouse brain have been reconstructed in their entirety. Here we present a robust and efficient platform for imaging and reconstructing complete neuronal morphologies, including axonal arbors that span substantial portions of the brain. We used this platform to reconstruct more than 1,000 projection neurons in the motor cortex, thalamus, subiculum, and hypothalamus. Together, the reconstructed neurons constitute more than 85 meters of axonal length and are available in a searchable online database. Axonal shapes revealed previously unknown subtypes of projection neurons and suggest organizational principles of long-range connectivity.

Development of Serotonergic Fibers in the Post-Natal Mouse Brain

Serotonin (5-HT)-synthetizing neurons, which are confined in the raphe nuclei of the rhombencephalon, provide a pervasive innervation of the central nervous system (CNS) and are involved in the modulation of a plethora of functions in both developing and adult brain. Classical studies have described the post-natal development of serotonergic axons as a linear process of terminal field innervation. However, technical limitations have hampered a fine morphological characterization. With the advent of genetic mouse models, the possibility to label specific neuronal populations allowed the rigorous measurement of their axonal morphological features as well as their developmental dynamics. Here, we used the Tph2^GFP knock-in mouse line, in which GFP expression allows punctual identification of serotonergic neurons and axons, for confocal microscope imaging and we performed 3-dimensional reconstruction in order to morphologically characterize the development of serotonergic fibers in specified brain targets from birth to adulthood. Our analysis highlighted region-specific developmental patterns of serotonergic fiber density ranging from a linear and progressive colonization of the target (Caudate/Putamen, Basolateral Amygdala, Geniculate Nucleus and Substantia Nigra) to a transient increase in fiber density (medial Prefrontal Cortex, Globus Pallidus, Somatosensory Cortex and Hippocampus) occurring with a region-specific timing. Despite a common pattern of early post-natal morphological maturation in which a progressive rearrangement from a dot-shaped to a regular and smooth fiber morphology was observed, starting from post-natal day 28 serotonergic fibers acquire the region specific morphological features present in the adult. In conclusion, we provided novel, target-specific insights on the morphology and temporal dynamics of the developing serotonergic fibers.

Postnatal Development of Intrinsic Horizontal Axons in Macaque Inferior Temporal and Primary Visual Cortices

Two distinct areas along the ventral visual stream of monkeys, the primary visual (V1) and inferior temporal (TE) cortices, exhibit different projection patterns of intrinsic horizontal axons with patchy terminal fields in adult animals. The differences between the patches in these 2 areas may reflect differences in cortical representation and processing of visual information. We studied the postnatal development of patches by injecting an anterograde tracer into TE and V1 in monkeys of various ages. At 1 week of age, labeled patches with distribution patterns reminiscent of those in adults were already present in both areas. The labeling intensity of patches decayed exponentially with projection distance in monkeys of all ages in both areas, but this trend was far less evident in TE. The number and extent of patches gradually decreased with age in V1, but not in TE. In V1, axonal and bouton densities increased postnatally only in patches with short projection distances, whereas in TE this density change occurred in patches with various projection distances. Thus, patches with area-specific distribution patterns are formed early in life, and area-specific postnatal developmental processes shape the connectivity of patches into adulthood.

The C-terminal binding protein (CTBP-1) regulates dorsal SMD axonal morphology in Caenorhabditis elegans

C-terminal binding proteins (CtBPs) are transcriptional co-repressors which cooperate with a variety of transcription factors to repress gene expression. Caenorhabditis elegans CTBP-1 expression has been observed in the nervous system and hypodermis. In C. elegans, CTBP-1 regulates several processes including Acute Functional Tolerance to ethanol and functions in the nervous system to modulate both lifespan and expression of a lipase gene called lips-7. Incorrect structure and/or function of the nervous system can lead to behavioral changes. Here, we demonstrate reduced exploration behavior in ctbp-1 mutants. Our examination of a subset of neurons involved in regulating locomotion revealed that the axonal morphology of dorsal SMD (SMDD) neurons is altered in ctbp-1 mutants at the fourth larval (L4) stage. Expressing CTBP-1 under the control of the endogenous ctbp-1 promoter rescued both the exploration behavior phenotype and defective SMDD axon structure in ctbp-1 mutants at the L4 stage. Interestingly, the pre-synaptic marker RAB-3 was found to localize to the mispositioned portion of SMDD axons in a ctbp-1 mutant. Further analysis of SMDD axonal morphology at days 1, 3 and 5 of adulthood revealed that the number of ctbp-1 mutants showing an SMDD axonal morphology defect increases in early adulthood and the observed defect appears to be qualitatively more severe. CTBP-1 is prominently expressed in the nervous system with weak expression detected in the hypodermis. Surprisingly, solely expressing CTBP-1a in the nervous system or hypodermis did not restore correct SMDD axonal structure in a ctbp-1 mutant. Our results demonstrate a role for CTBP-1 in exploration behavior and the regulation of SMDD axonal morphology in C. elegans.

From neural arbors to daisies

Pyramidal neurons in layers 2 and 3 of the neocortex collectively form an horizontal lattice of long-range, periodic axonal projections, known as the superficial patch system. The precise pattern of projections varies between cortical areas, but the patch system has nevertheless been observed in every area of cortex in which it has been sought, in many higher mammals. Although the clustered axonal arbors of single pyramidal cells have been examined in detail, the precise rules by which these neurons collectively merge their arbors remain unknown. To discover these rules, we generated models of clustered axonal arbors following simple geometric patterns. We found that models assuming spatially aligned but independent formation of each axonal arbor do not produce patchy labeling patterns for large simulated injections into populations of generated axonal arbors. In contrast, a model that used information distributed across the cortical sheet to generate axonal projections reproduced every observed quality of cortical labeling patterns. We conclude that the patch system cannot be built during development using only information intrinsic to single neurons. Information shared across the population of patch-projecting neurons is required for the patch system to reach its adult state.

The excitatory neuronal network of rat layer 4 barrel cortex

Sensory whiskers are mapped to rodent layer 4 somatosensory cortex as discrete units termed barrels, which can be visualized at high resolution in living brain slices. Both anatomical and physiological properties of the layer 4 neuronal network can thus be investigated in the context of the functional boundaries of this sensory map. Large-scale confinement of neuronal arbors to single barrels was suggested by restricted lateral diffusion of DiI across septa between barrels. Morphological analysis of dendritic and axonal arborizations of individual excitatory neurons showed that neuronal processes remain within the barrel of origin through polarization toward the center of the barrel. Functionally, the large-scale properties of the neuronal network were investigated through mapping the spatial extent of field EPSPs, which were found to attenuate at barrel borders. This ensemble property of a layer 4 barrel was further investigated by analyzing the connectivity of pairs of excitatory neurons with respect to the locations of the somata. Approximately one-third of the excitatory neurons within the same barrel were synaptically coupled. At the septum between adjacent barrels the connectivity dropped rapidly, and very few connections were found between neurons located in adjacent barrels. Each layer 4 barrel is thus composed of an excitatory neuronal network, which to a first order approximation, acts independently of its neighbors.