Biology subjects

Sahni, V.

Publications and source records attributed to Sahni, V..

5 recordsLinked to original sources

bioRxiv2022

Molecular specification of cortico-brainstem versus corticospinal projection neurons in development

Skilled motor control requires precise connections between subcerebral projection neurons (SCPN) in the cerebral cortex and their appropriate subcerebral targets in the brainstem or spinal cord. The brainstem is an important motor control center and cortical projections to the brainstem serve distinct motor control functions than corticospinal projections. However, mechanisms controlling cortico-brainstem versus corticospinal projections during development remain unknown. Here, we show that the transition between the brainstem and cervical cord distinguishes cortico-brainstem from corticospinal neurons from the earliest stages of SCPN axon extension in white matter. We used high throughput single-cell RNA sequencing of FACS-purified SCPN, retrogradely labeled from either the cerebral peduncle (labeling both cortico-brainstem and corticospinal neurons) or the cervical cord (labeling corticospinal neurons only) at critical times of axon extension. We identify that cortico-brainstem and corticospinal neurons are molecularly distinct: We establish Neuropeptide Y (Npy) as specifically enriched in cortico-brainstem neurons in lateral cortex, while CART prepropeptide (Cartpt) delineates cervical-projecting corticospinal neurons. Our results highlight molecular specification of cortico-brainstem vs. corticospinal projections well before these axons reach their appropriate segmental target and suggest a broad molecular program over SCPN axon targeting to distinct subcerebral targets early in development. These findings are likely to inform future investigations of motor circuit development, as well as approaches aimed at enhancing motor recovery after central nervous system damage. HighlightsO_LICortico-brainstem neurons (CBN) limit their axon extension to supraspinal levels from the earliest time points of white matter axon extension in development. C_LIO_LICBN can be molecularly delineated from corticospinal neurons (CSN) even at these initial time points. C_LIO_LIMolecular diversification of developing subcerebral projection neurons occurs across at least two axes: cortical location (medial vs. lateral) and projection targeting specificity (brainstem vs. spinal) C_LIO_LIWithin lateral cortex, Neuropeptide Y (Npy) is expressed by CBN, while CART prepropeptide (Cartpt) expression delineates cervical-projecting CSN. C_LI O_FIG O_LINKSMALLFIG WIDTH=146 HEIGHT=200 SRC="FIGDIR/small/494253v1_ufig1.gif" ALT="Figure 1"> View larger version (38K): org.highwire.dtl.DTLVardef@48776corg.highwire.dtl.DTLVardef@1987eaeorg.highwire.dtl.DTLVardef@a50390org.highwire.dtl.DTLVardef@18d1588_HPS_FORMAT_FIGEXP M_FIG C_FIG

neuroscience↗

bioRxiv2022

Cbln1 directs axon targeting by corticospinal neurons specifically toward thoraco-lumbar spinal cord.

Corticospinal neurons (CSN) are centrally required for skilled voluntary movement, which necessitates that they establish precise subcerebral connectivity with the brainstem and spinal cord. However, molecular controls regulating specificity of this projection targeting remain largely unknown. We previously identified that developing CSN subpopulations exhibit striking axon targeting specificity in the spinal white matter. These CSN subpopulations with segmentally distinct spinal projections are also molecularly distinct; a subset of differentially expressed genes between these distinct CSN subpopulations function as molecular controls regulating differential axon projection targeting. Rostrolateral CSN extend axons exclusively to bulbar-cervical segments (CSNBC-lat), while caudomedial CSN (CSNmedial) are more heterogeneous, with distinct, intermingled subpopulations extending axons to either bulbar-cervical or thoraco-lumbar segments. Here, we report that Cerebellin 1 (Cbln1) is expressed specifically by CSN in medial, but not lateral, sensorimotor cortex. Cbln1 shows highly dynamic temporal expression, with Cbln1 levels in CSN highest during the period of peak axon extension toward thoraco-lumbar segments. Using gain-of-function experiments, we identify that Cbln1 is sufficient to direct thoraco-lumbar axon extension by CSN. Mis-expression of Cbln1 in CSNBC-lat either by in utero electroporation, or in postmitotic CSNBC-lat by AAV-mediated gene delivery, re-directs these axons past their normal bulbar-cervical targets toward thoracic segments. Further, Cbln1 overexpression in postmitotic CSNmedial increases the number of CSNmedial axons that extend past cervical segments into the thoracic cord. Collectively, these results identify that Cbln1 functions as a potent molecular control over thoraco-lumbar CSN axon extension, part of an integrated network of controls over segmentally-specific CSN axon projection targeting. Significance StatementCorticospinal neurons (CSN) exhibit remarkable diversity and precision of axonal projections to targets in the brainstem and distinct spinal segments; the molecular basis for this targeting diversity is largely unknown. CSN subpopulations projecting to distinct targets are also molecularly distinguishable. Distinct subpopulations degenerate in specific motor neuron diseases, further suggesting that intrinsic molecular differences might underlie differential vulnerability to disease. Here, we identify a novel molecular control, Cbln1, expressed by CSN extending axons to thoraco-lumbar spinal segments. Cbln1 is sufficient, but not required, for CSN axon extension toward distal spinal segments, and Cbln1 expression is controlled by recently identified, CSN-intrinsic regulators of axon extension. Our results identify that Cbln1, together with other regulators, coordinates segmentally precise CSN axon targeting.

neuroscience↗

bioRxiv2022

Long-distance axon growth ability of corticospinal neurons is lost in a segmentally-distinct manner.

Established experimental lesions in the developing central nervous system (CNS) disrupt microenvironments critical for long-distance axon growth and guidance. Therefore, the precise developmental time at which the CNS ceases its capacity to support long-distance axon growth remains unknown. Accordingly, we established a new microsurgical approach to axotomize developing corticospinal tract (CST) axons in the neonatal spinal cord while leaving their local microenvironments relatively intact. This enables unambiguous investigation of long-distance CST growth ability in the CNS. Surprisingly, we find that CST axons lose their capacity for long-distance growth even during the developmental period of CST extension. While this ability remains intact in spinal locations where normal CST extension is occurring, it is completely abolished at sites distant from these locations. Further, the developmental time window for which this ability is maintained is much shorter than for other forms of axon growth such as sprouting. Long-distance CST growth ability does not correlate with astrocytic or microglial activation, nor with myelination levels. These results indicate that long-distance CST growth is controlled by mechanisms that operate early in development in a time- and region-specific manner.

neuroscience↗

bioRxiv2021

Lumican regulates cervical corticospinal axon collateralization via non-autonomous crosstalk between distinct corticospinal neuron subpopulations

Corticospinal neurons (CSN) are the cortical projection neurons that innervate the spinal cord and some brainstem targets with segmental precision to control voluntary movement of specific functional motor groups, limb sections, or individual digits. CSN subpopulations exhibit striking axon targeting specificity from development into maturity: Evolutionarily newer rostrolateral CSN exclusively innervate bulbar-cervical targets (CSNBC-lat), while evolutionarily older caudomedial CSN (CSNmedial) are more heterogeneous, with distinct subpopulations extending axons to either bulbar-cervical or thoraco-lumbar segments. However, molecular regulation over specificity of CSN segmental target innervation is essentially unknown. The cervical cord, with its evolutionarily enhanced precision of forelimb movement, is innervated by multiple CSN subpopulations, suggesting inter-neuronal interactions in establishing cervical corticospinal circuitry. Here, we identify that Lumican, previously unrecognized in axon development, controls the balance of innervation between CSNBC-lat and CSNmedial within the cervical spinal cord. Remarkably, Lumican, an extracellular matrix protein expressed by CSNBC-lat, non-cell-autonomously suppresses axon collateralization in the cervical cord by CSNmedial. Intersectional viral labeling and mouse genetics further identify that Lumican controls axon collateralization by multiple CSN subpopulations in caudomedial sensorimotor cortex. These results identify inter-axonal molecular crosstalk between CSN subpopulations as a novel mechanism controlling corticospinal circuitry, target density, and competitive specificity. Further, this mechanism has potential implications for evolutionary diversification of corticospinal circuitry with finer scale precision.

neuroscience↗

bioRxiv2020

An evolutionarily acquired microRNA shapes development of mammalian cortical projections

The corticospinal tract is unique to mammals and the corpus callosum is unique to placental mammals (eutherians). The emergence of these structures is thought to underpin the evolutionary acquisition of complex motor and cognitive skills. Corticospinal motor neurons (CSMN) and callosal projection neurons (CPN) are the archetypal projection neurons of the corticospinal tract and corpus callosum, respectively. Although a number of conserved transcriptional regulators of CSMN and CPN development have been identified in vertebrates, none are unique to mammals and most are co-expressed across multiple projection neuron subtypes. Here, we discover seventeen CSMN-enriched microRNAs (miRNAs), fifteen of which map to a single genomic cluster that is exclusive to eutherians. One of these, miR-409-3p, promotes CSMN subtype identity in part via repression of LMO4, a key transcriptional regulator of CPN development. In vivo, miR-409-3p is sufficient to convert deep-layer CPN into CSMN. This is the first demonstration of an evolutionarily acquired miRNA in eutherians that refines cortical projection neuron subtype development. Our findings implicate miRNAs in the eutherians increase in neuronal subtype and projection diversity, the anatomic underpinnings of their complex behavior. Significance StatementThe mammalian central nervous system contains unique projections from the cerebral cortex thought to underpin complex motor and cognitive skills, including the corticospinal tract and corpus callosum. The neurons giving rise to these projections - corticospinal and callosal projection neurons - develop from the same progenitors, but acquire strikingly different fates. The broad evolutionary conservation of known genes controlling cortical projection neuron fates raises the question of how the more narrowly conserved corticospinal and callosal projections evolved. We identify a microRNA cluster selectively expressed by corticospinal projection neurons and exclusive to placental mammals. One of these microRNAs promotes corticospinal fate via regulation of the callosal gene LMO4, suggesting a mechanism whereby microRNA regulation during development promotes evolution of neuronal diversity.

neuroscience↗