Abnormal Pyramidal Decussation and Bilateral Projection of the Corticospinal Tract Axons in Mice Lacking the Heparan Sulfate Endosulfatases, Sulf1 and Sulf2

The diversity and plasticity of descending motor pathways rewired after stroke and trauma in rodents

Descending neural pathways to the spinal cord plays vital roles in motor control. They are often damaged by brain injuries such as stroke and trauma, which lead to severe motor impairments. Due to the limited capacity for regeneration of neural circuits in the adult central nervous system, currently no essential treatments are available for complete recovery. Notably, accumulating evidence shows that residual circuits of the descending pathways are dynamically reorganized after injury and contribute to motor recovery. Furthermore, recent technological advances in cell-type classification and manipulation have highlighted the structural and functional diversity of these pathways. Here, we focus on three major descending pathways, namely, the corticospinal tract from the cerebral cortex, the rubrospinal tract from the red nucleus, and the reticulospinal tract from the reticular formation, and summarize the current knowledge of their structures and functions, especially in rodent models (mice and rats). We then review and discuss the process and patterns of reorganization induced in these pathways following injury, which compensate for lost connections for recovery. Understanding the basic structural and functional properties of each descending pathway and the principles of the induction and outcome of the rewired circuits will provide therapeutic insights to enhance interactive rewiring of the multiple descending pathways for motor recovery.

Spontaneously regenerative corticospinal neurons in mice

The spinal cord receives inputs from the cortex via corticospinal neurons (CSNs). While predominantly a contralateral projection, a less-investigated minority of its axons terminate in the ipsilateral spinal cord. We analyzed the spatial and molecular properties of these ipsilateral axons and their post-synaptic targets in mice and found they project primarily to the ventral horn, including directly to motor neurons. Barcode-based reconstruction of the ipsilateral axons revealed a class of primarily bilaterally-projecting CSNs with a distinct cortical distribution. The molecular properties of these ipsilaterally-projecting CSNs (IP-CSNs) are strikingly similar to the previously described molecular signature of embryonic-like regenerating CSNs. Finally, we show that IP-CSNs are spontaneously regenerative after spinal cord injury. The discovery of a class of spontaneously regenerative CSNs may prove valuable to the study of spinal cord injury. Additionally, this work suggests that the retention of juvenile-like characteristics may be a widespread phenomenon in adult nervous systems.

Knockout of the intellectual disability-linked gene Hs6st2 in mice decreases heparan sulfate 6-O-sulfation, impairs dendritic spines of hippocampal neurons, and affects memory

Heparan sulfate (HS) is a linear polysaccharide that plays a key role in cellular signaling networks. HS functions are regulated by its 6-O-sulfation, which is catalyzed by three HS 6-O-sulfotransferases (HS6STs). Notably, HS6ST2 is mainly expressed in the brain and HS6ST2 mutations are linked to brain disorders, but the underlying mechanisms remain poorly understood. To determine the role of Hs6st2 in the brain, we carried out a series of molecular and behavioral assessments on Hs6st2 knockout mice. We first carried out strong anion exchange-high performance liquid chromatography and found that knockout of Hs6st2 moderately decreases HS 6-O-sulfation levels in the brain. We then assessed body weights and found that Hs6st2 knockout mice exhibit increased body weight, which is associated with abnormal metabolic pathways. We also performed behavioral tests and found that Hs6st2 knockout mice showed memory deficits, which recapitulate patient clinical symptoms. To determine the molecular mechanisms underlying the memory deficits, we used RNA sequencing to examine transcriptomes in two memory-related brain regions, the hippocampus and cerebral cortex. We found that knockout of Hs6st2 impairs transcriptome in the hippocampus, but only mildly in the cerebral cortex. Furthermore, the transcriptome changes in the hippocampus are enriched in dendrite and synapse pathways. We also found that knockout of Hs6st2 decreases HS levels and impairs dendritic spines in hippocampal CA1 pyramidal neurons. Taken together, our study provides novel molecular and behavioral insights into the role of Hs6st2 in the brain, which facilitates a better understanding of HS6ST2 and HS-linked brain disorders.

Coordinating cerebral cortical construction and connectivity: Unifying influence of radial progenitors

Radial progenitor development and function lay the foundation for the construction of the cerebral cortex. Radial glial scaffold, through its functions as a source of neurogenic progenitors and neuronal migration guide, is thought to provide a template for the formation of the cerebral cortex. Emerging evidence is challenging this limited view. Intriguingly, radial glial scaffold may also play a role in axonal growth, guidance, and neuronal connectivity. Radial glial cells not only facilitate the generation, placement, and allocation of neurons in the cortex but also regulate how they wire up. The organization and function of radial glial cells may thus be a unifying feature of the developing cortex that helps to precisely coordinate the right patterns of neurogenesis, neuronal placement, and connectivity necessary for the emergence of a functional cerebral cortex. This perspective critically explores this emerging view and its impact in the context of human brain development and disorders.

Parcellation of the murine cortical hindlimb area is demonstrated by its subcortical connectivity and cytoarchitecture

Although corticospinal neurons are known to be distributed in both the primary motor and somatosensory cortices (S1), details of the projection pattern of their fibers to the lumbar cord gray matter remain largely uncharacterized, especially in rodents. We previously investigated the cortical area projecting to the gray matter of the fourth lumbar cord segment (L4) (L4 Cx) in mice. In the present study, we injected an anterograde tracer into multiple sites to cover the entire L4 Cx. We found that (1) the rostromedial part of the L4 Cx projects to the intermediate and ventral zones of the lumbar cord gray matter, (2) the lateral part projects to the medial dorsal horn, and (3) the caudal part projects to the lateral dorsal horn. We also found that the border between the rostromedial and caudolateral parts corresponds to the border between the agranular and granular cortex. Analysis of the somatotopic patterns formed by the cortical projection cells and the primary sensory neurons innervating the skin of the hindlimb and its related area suggests that the lateral part corresponds to the S1 hindlimb area and the caudal part to the S1 trunk area. Examination of thalamic innervation by the L4 Cx revealed that the caudolateral L4 Cx focally projects to the ventrobasal complex (VB) and the posterior complex (PO), while the medial L4 Cx widely projects to the PO but little to the VB. These findings suggest that the L4 Cx is parceled into subregions defined by the cytoarchitecture and subcortical projection.

Physiology and Pathophysiology of Heparan Sulfate in Animal Models: Its Biosynthesis and Degradation

Heparan sulfate (HS) is a type of glycosaminoglycan that plays a key role in a variety of biological functions in neurology, skeletal development, immunology, and tumor metastasis. Biosynthesis of HS is initiated by a link of xylose to Ser residue of HS proteoglycans, followed by the formation of a linker tetrasaccharide. Then, an extension reaction of HS disaccharide occurs through polymerization of many repetitive units consisting of iduronic acid and N-acetylglucosamine. Subsequently, several modification reactions take place to complete the maturation of HS. The sulfation positions of N-, 2-O-, 6-O-, and 3-O- are all mediated by specific enzymes that may have multiple isozymes. C5-epimerization is facilitated by the epimerase enzyme that converts glucuronic acid to iduronic acid. Once these enzymatic reactions have been completed, the desulfation reaction further modifies HS. Apart from HS biosynthesis, the degradation of HS is largely mediated by the lysosome, an intracellular organelle with acidic pH. Mucopolysaccharidosis is a genetic disorder characterized by an accumulation of glycosaminoglycans in the body associated with neuronal, skeletal, and visceral disorders. Genetically modified animal models have significantly contributed to the understanding of the in vivo role of these enzymes. Their role and potential link to diseases are also discussed.

Regulation of fractone heparan sulfate composition in young and aged subventricular zone neurogenic niches

Fractones, specialized extracellular matrix structures found in the subventricular zone (SVZ) neurogenic niche, can capture growth factors, such as basic fibroblast growth factor, from the extracellular milieu through a heparin-binding mechanism for neural stem cell presentation, which promotes neurogenesis. During aging, a decline in neurogenesis correlates with a change in the composition of heparan sulfate (HS) within fractones. In this study, we used antibodies that recognize specific short oligosaccharides with varying sulfation to evaluate the HS composition in fractones in young and aged brains. To further understand the conditions that regulate 6-O sulfation levels and its impact on neurogenesis, we used endosulfatase Sulf1 and Sulf2 double knock out (DKO) mice. Fractones in the SVZ of Sulf1/2 DKO mice showed immunoreactivity for the HS epitope, suggesting higher 6-O sulfation. While neurogenesis declined in the aged SVZ of both WT and Sulf1/2 DKO mice, we observed a larger number of neuroblasts in the young and aged SVZ of Sulf1/2 DKO mice. Together, these results show that the removal of 6-O-sulfation in fractones HS by endosulfatases inhibits neurogenesis in the SVZ. Our findings advance the current understanding regarding the extracellular environment that is best suited for neural stem cells to thrive, which is critical for the design of future stem cell therapies.