A Hox Code Defines Spinocerebellar Neuron Subtype Regionalization

Hox genes and patterning the vertebrate body

The diversity of vertebrate body plans is dizzying, yet stunning for the many things they have in common. Vertebrates have inhabited virtually every part of the earth from its coldest to warmest climates. They locomote by swimming, flying, walking, slithering, or climbing, or combinations of these behaviors. And they exist in many different sizes, from the smallest of frogs, fish and lizards to giraffes, elephants, and blue whales. Despite these differences, vertebrates follow a remarkably similar blueprint for the establishment of their body plan. Within the relatively small amount of time required to complete gastrulation, the process through which the three germ layers, ectoderm, mesoderm, and endoderm are created, the embryo also generates its body axis and is simultaneously patterned. For the length of this axis, the genes that distinguish the neck from the rib cage or the trunk from the sacrum are the Hox genes. In vertebrates, there was evolutionary pressure to maintain this set of genes in the organism. Over the past decades, much has been learned regarding the regulatory mechanisms that ensure the appropriate expression of these genes along the main body axes. Genetic functions continue to be explored though much has been learned. Much less has been discerned on the identity of co-factors used by Hox proteins for the specificity of transcriptional regulation or what downstream targets and pathways are critical for patterning events, though there are notable exceptions. Current work in the field is demonstrating that Hox genes continue to function in many organs long after directing early patterning events. It is hopeful continued research will shed light on remaining questions regarding mechanisms used by this important and conserved set of transcriptional regulators.

The pelvic organs receive no parasympathetic innervation

The pelvic organs (bladder, rectum, and sex organs) have been represented for a century as receiving autonomic innervation from two pathways - lumbar sympathetic and sacral parasympathetic - by way of a shared relay, the pelvic ganglion, conceived as an assemblage of sympathetic and parasympathetic neurons. Using single-cell RNA sequencing, we find that the mouse pelvic ganglion is made of four classes of neurons, distinct from both sympathetic and parasympathetic ones, albeit with a kinship to the former, but not the latter, through a complex genetic signature. We also show that spinal lumbar preganglionic neurons synapse in the pelvic ganglion onto equal numbers of noradrenergic and cholinergic cells, both of which therefore serve as sympathetic relays. Thus, the pelvic viscera receive no innervation from parasympathetic or typical sympathetic neurons, but instead from a divergent tail end of the sympathetic chains, in charge of its idiosyncratic functions.

Timing and precision of rattlesnake spinal motoneurons are determined by the KV72/3 potassium channel

The evolution of novel motor behaviors requires modifications in the central pattern generators (CPGs) controlling muscle activity. How such changes gradually lead to novel behaviors remains enigmatic due to the long time course of evolution. Rattlesnakes provide a unique opportunity to investigate how a locomotor CPG was evolutionarily modified to generate a novel behavior-in this case, acoustic signaling. We show that motoneurons (MNs) in the body and tail spinal cord of rattlesnakes possess fundamentally different physiological characteristics, which allow MNs in the tail to integrate and transmit CPG output for controlling superfast muscles with high temporal precision. Using patch-clamp electrophysiology, we demonstrate that these differences in locomotor and rattle MNs are mainly determined by KV72/3 potassium channels. However, although KV72/3 exerted a significantly different influence on locomotor and rattle MN physiology, single-cell RNA-seq unexpectedly did not reveal any differences in KV72/3 channels' expression. VIDEO ABSTRACT.

BAF45D-binding to HOX genes was differentially targeted in H9-derived spinal cord neural stem cells

Chromatin accessibility has been used to define how cells adopt region-specific neural fates. BAF45D is one of the subunits of a specialised chromatin remodelling BAF complex. It has been reported that BAF45D is expressed in spinal cord neural stem cells (NSCs) and regulates their fate specification. Within the developing vertebrate spinal cord, HOX genes exhibit spatially restricted expression patterns. However, the chromatin accessibility of BAF45D binding HOX genes in spinal cord NSCs is unclear. In the present study, we found that in H9-derived spinal cord NSCs, BAF45D targets TBX6, a gene that regulates spinal cord neural mesodermal progenitors. Furthermore, BAF45D binding to the NES gene is much more enriched in H9-derived spinal cord NSCs chromatin compared to ESCs chromatin. In addition, BAF45D binding to anterior and trunk/central HOX genes, but not to lumbosacral HOX genes, was much more enriched in NSCs chromatin compared to ESCs chromatin. These results may shed new light on the role of BAF45D in regulating region-specific spinal cord NSCs by targeting HOX genes.

Hox genes in development and beyond

Hox genes encode evolutionarily conserved transcription factors that are essential for the proper development of bilaterian organisms. Hox genes are unique because they are spatially and temporally regulated during development in a manner that is dictated by their tightly linked genomic organization. Although their genetic function during embryonic development has been interrogated, less is known about how these transcription factors regulate downstream genes to direct morphogenetic events. Moreover, the continued expression and function of Hox genes at postnatal and adult stages highlights crucial roles for these genes throughout the life of an organism. Here, we provide an overview of Hox genes, highlighting their evolutionary history, their unique genomic organization and how this impacts the regulation of their expression, what is known about their protein structure, and their deployment in development and beyond.

Maintenance of neurotransmitter identity by Hox proteins through a homeostatic mechanism

Hox transcription factors play fundamental roles during early patterning, but they are also expressed continuously, from embryonic stages through adulthood, in the nervous system. However, the functional significance of their sustained expression remains unclear. In C. elegans motor neurons (MNs), we find that LIN-39 (Scr/Dfd/Hox4-5) is continuously required during post-embryonic life to maintain neurotransmitter identity, a core element of neuronal function. LIN-39 acts directly to co-regulate genes that define cholinergic identity (e.g., unc-17/VAChT, cho-1/ChT). We further show that LIN-39, MAB-5 (Antp/Hox6-8) and the transcription factor UNC-3 (Collier/Ebf) operate in a positive feedforward loop to ensure continuous and robust expression of cholinergic identity genes. Finally, we identify a two-component design principle for homeostatic control of Hox gene expression in adult MNs: Hox transcriptional autoregulation is counterbalanced by negative UNC-3 feedback. These findings uncover a noncanonical role for Hox proteins during post-embryonic life, critically broadening their functional repertoire from early patterning to the control of neurotransmitter identity.

In-vitro differentiation of human embryonic stem cells into spinal cord neural stem cells

In-vitro differentiation of human embryonic stem cells into spinal cord neural stem cells (NSCs) can help researchers better understand the cellular processes associated with spinal cord development and regeneration, and provide therapeutic strategies for spinal cord disorders. However, effective and consistent methods for the generation of human spinal cord NSCs are rare. Objective of the study is to establish methods for the in-vitro induction and long-term maintenance of human spinal cord NSCs. H9 cells were treated with neural induction medium for 10 days under single-cell seeding condition, followed by treatment with neural maintenance medium and replacement with NSC medium after five passages. The identity of the generated cells was determined by immunofluorescence, immunoblotting, and cleavage under targets and tagmentation (CUT&Tag) assays. After the neural induction, OCT4, an embryonic stem cell marker, was significantly reduced, whereas NESTIN and PAX6, two NSC markers, were clearly increased. After the neural maintenance, most of the H9-derived cells consistently expressed NESTIN and PAX6 together with SOX1 and HOXC9, two spinal cord markers. The Homer known motif enrichment results of the CUT&Tag assay confirmed the expression of HOXC9 in the H9-derived spinal cord NSCs, which can be maintained for more than 40 days under an in vitro culture system. This study sheds new light on effective induction and maintenance of human spinal cord NSCs.

Emerging Roles for Hox Proteins in the Last Steps of Neuronal Development in Worms, Flies, and Mice

A remarkable diversity of cell types characterizes every animal nervous system. Previous studies provided important insights into how neurons commit to a particular fate, migrate to the right place and form precise axodendritic patterns. However, the mechanisms controlling later steps of neuronal development remain poorly understood. Hox proteins represent a conserved family of homeodomain transcription factors with well-established roles in anterior-posterior (A-P) patterning and the early steps of nervous system development, including progenitor cell specification, neuronal migration, cell survival, axon guidance and dendrite morphogenesis. This review highlights recent studies in Caenorhabditis elegans, Drosophila melanogaster and mice that suggest new roles for Hox proteins in processes occurring during later steps of neuronal development, such as synapse formation and acquisition of neuronal terminal identity features (e.g., expression of ion channels, neurotransmitter receptors, and neuropeptides). Moreover, we focus on exciting findings suggesting Hox proteins are required to maintain synaptic structures and neuronal terminal identity during post-embryonic life. Altogether, these studies, in three model systems, support the hypothesis that certain Hox proteins are continuously required, from early development throughout post-embryonic life, to build and maintain a functional nervous system, significantly expanding their functional repertoire beyond the control of early A-P patterning.

Breaking constraint of mammalian axial formulae

The vertebral column of individual mammalian species often exhibits remarkable robustness in the number and identity of vertebral elements that form (known as axial formulae). The genetic mechanism(s) underlying this constraint however remain ill-defined. Here, we reveal the interplay of three regulatory pathways (Gdf11, miR-196 and Retinoic acid) is essential in constraining total vertebral number and regional axial identity in the mouse, from cervical through to tail vertebrae. All three pathways have differing control over Hox cluster expression, with heterochronic and quantitative changes found to parallel changes in axial identity. However, our work reveals an additional role for Hox genes in supporting axial elongation within the tail region, providing important support for an emerging view that mammalian Hox function is not limited to imparting positional identity as the mammalian body plan is laid down. More broadly, this work provides a molecular framework to interrogate mechanisms of evolutionary change and congenital anomalies of the vertebral column.

Vertebral column length and shape exhibits remarkable robustness within a species but diversity across species. Here the authors reveal the molecular logic constraining vertebral number in mouse and a novel role for posterior Hox genes in this context.

Ascending spinal tract formation in chick embryo originating from different spinal regions

The present study aimed to assess spinal tract formation in neurons originating from cervical (C7), brachial (C14), and thoracic (T4) regions, with the lumbar (LS2) region as a reference, in a chick embryo. For the assessment of the spinal tracts, we introduced a vector expressing human placental alkaline phosphatase into progenitor cells generated after neural tube closure and belonging to the above segments, using in ovo electroporation. The ascending axons took primarily similar paths: dorsal commissural, ventral commissural, and dorsal non-commissural paths, with some variance depending on their originating segments. Some populations of non-commissural neurons later extended their axons following a ventral path. The elongation rates of these axons are primarily constant and tended to increase over time; however, some variations depending on the originating segments were also observed. Some of the dorsally ascending axons entered into the developing cerebellum, and spinocerebellar neurons originating from T4 projected their axons into the cortex of the cerebellum differently from those from LS2. These results unveil an overall picture of early ascending spinal tract formation.

HOX genes and the NF-κB pathway: A convergence of developmental biology, inflammation and cancer biology

The roles of HOX transcription factors as oncogenes and tumor suppressor genes, and the NF-KB pathway in chronic inflammation, both leading to cancer are well-established. HOX transcription factors are members of an evolutionarily conserved family of proteins required for anteroposterior body axis patterning during embryonic development, and are often dysregulated in cancer. The NF-KB pathway aids inflammation and immunity but it is also important during embryonic development. It is frequently activated in both solid and hematological malignancies. NF-KB and HOX proteins can influence each other through mutual transcriptional regulation, protein-protein interactions, and regulation of upstream and downstream interactors. These interactions have important implications both in homeostasis and in disease. In this review, we summarize the role of HOX proteins in regulating inflammation in homeostasis and disease- with a particular emphasis on cancer. We also describe the relationship between HOX genes and the NF-KB pathway, and discuss potential therapeutic strategies.