Neural Development PI Note PDF

Yong Lu, Ph.D., Neural Development, PI note, M1 B&B Session Objectives: 1. Compare and contrast the two basic types of circuits (hierarchical and single-source divergent circuits). 2. Describe the basic steps in development of circuits and synaptic connections. 3. Compare neurog...

Yong Lu, Ph.D., Neural Development, PI note, M1 B&B Session Objectives: 1. Compare and contrast the two basic types of circuits (hierarchical and single-source divergent circuits). 2. Describe the basic steps in development of circuits and synaptic connections. 3. Compare neurogenesis and migration in the spinal cord and the cerebral cortex. 4. List the neurotrophins and their receptors and functions. 5. Describe postnatal development and define Hebbian Rule. 6. List brain disorders caused by mutation of genes: DCX, SHH, Robo3, VLDLR. Overview Statement: “wiring” of the nervous system takes many steps and involves a great number of signaling molecules, mutation of which gives rise to neurodevelopment disorders. Preparatory Materials (Required Reading): PI note Pre-recorded video Supplemental Materials: Chapters 23 and 24 of Augustine et al., 7th ed (2023). I. Hierarchical-chain vs Single-source divergent circuits Unlike cells of liver or kidney or other organs, individual neurons do not perform local functions in isolated units. The billions of neurons in the nervous system work by receiving messages from, and sending messages to other neurons in linked pathways called Circuit. There are only a few basic patterns of circuits (with many minor variations). Hierarchical-chain circuits. Most common type. Characteristic of sensory and motor pathways. Typical sensory systems are organized as ascending hierarchies. Typical motor systems are organized as descending hierarchies. Chain circuits are vulnerable to interference by injury, disease, stroke or tumor, which leaves the whole circuit/pathway inoperative. Interneurons are interposed in these chains. Interneurons have short axons and operate in a spatially restricted area, forming local circuits. They are either excitatory or inhibitory, or modulatory, modulating flow of information in pathways. Effects of lesion of the circuit are specific (e.g., loss of sensation, or paralysis of certain muscles in a motor pathway). Single-source divergent circuits Neurons send axons to many recipient cells (e.g., throughout cerebral cortex) and their effects are extremely divergent. Influence large numbers of target cells at many levels simultaneously. Participate in “volume” neurotransmission (extrasynaptic) vs. “wired” neurotransmission (synaptic), i.e., most of their axon terminals “dump” neurotransmitters into the extracellular space, which diffuse to many target neurons. E.g., neurons in raphe nuclei release serotonin to various targets all over the cerebral and cerebellar cortex. 1  Yong Lu, Ph.D., Neural Development, PI note, M1 B&B Effects produced by these circuits are “nonspecific” (e.g., mood, arousal) compared to “specific” effects of hierarchical/chain circuits. Effects of transmitters are “conditional”; they depend on existing conditions in the area of release and influence global behavioral states, e.g., changes in mood, arousal, attentiveness etc. Lesion of such circuits produces nonspecific effects (changes in mood, pleasure, reward, personality, depression, disrupted sleep, etc.), rather than a specific loss of a function (loss of pain, paralysis of muscles) that one would see with a hierarchical/chain circuit. II. Development of circuits embryologically The nervous system develops via many interrelated steps. A. Neurogenesis: cell proliferation, birth of neurons and glia. B. Migration: newly formed cells relocate. C. Aggregation & Differentiation. Aggregation: similar neurons that are destined to function together form groups as nuclei in the adult. Differentiation: finer delineation of neurons to specific characters, e.g., neurotransmitter phenotypes. D. Synaptogenesis & Circuit Formation: formation of synapses and connection genesis. E. Apoptosis: programmed cell death of overproduced neurons. F. Synapse Rearrangement: maturation of synapses and connections via strengthening some synapses while weakening others. A & B. Neurogenesis and Migration Cell proliferation dominates in early development. The wall of the neural tube in later development consists of three layers: neuroepithelial layer (ventricular zone). The neuroepithelial (neuroectodermal) cells of the ventricular zone are mitotically active throughout intrauterine development, and are the sources of all neurons and astrocytes (& maybe some oligodendrocytes) of the CNS. Neuroepithelial cells divide and form neuroblasts (primitive/immature nerve cells) and glioblasts (primitive/immature glial cells). After all the cells enter a postmitotic state, the remaining cells form the ependyma lining the ventricular system. mantle layer (intermediate zone) consists of neuroblasts and glioblasts. Cells in mantle layer will form grey matter for spinal cord. marginal layer consists of axons originating from neurons in the mantle layer, and axons that pass through from lower and/or higher centers, forming white matter for spinal cord. Neurogenesis and migration in the spinal cord Neural migration follows distinctive patterns in different regions of the neural tube. In the spinal cord, neuroblasts migrate into the mantle zone, leaving the marginal zone as the site for axonal pathways. Therefore, in the adult, grey matter is on the inside, and white matter outside. Netter’s Atlas of Human Neuroscience, Figs I.69, I.70 2  Yong Lu, Ph.D., Neural Development, PI note, M1 B&B Neurogenesis and migration in the cerebral cortex Grey matter of the cerebral cortex in the adult is on the outside of the brain. Cells proliferate in the ventricular zone, migrate and pass the subplate, forming layer VI of cortex. Subplate neurons are among the first generated neurons, and disappear during postnatal development. They are important in establishing correct wiring in the cortex. Newly born cells migrate through the first layer, forming additional cortical layers. Newer cells are closer to surface (inside-out pattern of development). Axons leaving and coming into the cortex form white matter of the cortex (in the original mantle zone). The marginal zone becomes Layer I (few cell bodies, lots of processes). The outer layers of the cortex are associational, last to develop, least vascularized, and most susceptible to damage. Damage can lead to mental retardation, cerebral palsy etc. Migration: how? Neurons migrate along the processes of specialized glial cells: radial glial cells. This type of migration occurs for neurons that are destined to form a laminated or semi-laminated structure (e.g. cerebral and cerebellar cortices, hippocampus, spinal cord). The cell bodies of the radial glial cells are located in the ventricular zone, and their processes extend all the way through the mantle zone and into the marginal zone. The elongated migrating neurons (labeled green in panel A with antibody against neuregulin) form close associations with glial processes as they crawl along them. The leading processes of migrating cells are highly active, extending numerous filopodia which move along the processes of the radial glial cells. Clinical note: Mutations in migration genes cause malformation of cerebral cortex. In Reelin (a protein at the glial endfoot which signals the neuron to detach from the glia) mutation, lateral ventricles are enlarged, white matter is diminished, and the pattern of sulci and gyri is disrupted. In doublecortin (DCX, a protein that interacts with microtubes to keep the integrity of the cytoskeleton) mutation, the ventricles are dramatically enlarged, little subcortical white matter is present, and no sulci and gyri (lissencephaly: smooth brain). The leading process and the trailing process of migrating neurons are transient structures; they disappear before they differentiate in later development. Other neurons that are destined to form non-laminated structures migrate without the benefit of radial glial guides. Rather, they migrate by using tissue, cellular and molecular signals as cues. 3  Yong Lu, Ph.D., Neural Development, PI note, M1 B&B C. Cell Aggregation and Differentiation Neuroblasts arise by division of the neuroepithelial cells. When they migrate, they have transient structure (leading process and trailing process). After they migrate, these processes disappear, and the neuroblasts are temporarily round (apolar: lack of polarity). With differentiation, two cytoplasmic processes appear on opposite sides of the cell body (neuronal polarization), forming a bipolar neuroblast. The process at one end elongates rapidly to form the primitive axon, and the other shows a number of cytoplasmic arborizations, forming the primitive dendrites. The cell is known as a multipolar neuroblast, which with further development will become a neuron with varying morphology depending on its location in the NS. Glioblasts (primitive glial cells) are also formed by neuroepithelial cells. They migrate to the mantle and marginal layers and differentiate into astrocytes (protoplasmic astrocytes in grey matter, fibrillar/fibrous astrocytes in white matter). In the marginal layer, they possibly also become oligodendrocytes. Oligodendrocytes can also derive from mesenchyme cells. Microglia develop from mesoderm or neuroectoderm (neural tube and neural crest); their appearance coincides with vascularization of the CNS. Cells generated during neurogenesis are deemed to develop into glial cells in default, but their fates are changed by different molecules via gene expression and cell-cell signaling. Various genes control when and what the precursor cells become (you need not to memorize the signaling genes). Sonic hedgehog (Shh) and neurodevelopmental disorders Shh, a peptide hormone, is produced by cells in the Floor Plate. It is important in establishing the identity of neurons. The signaling pathway of Shh involves two membrane receptors, Patched protein and Smoothened, and a transcription factor (Gli1) regulating gene expression. Clinical note: Mutation in SHH is associated with diseases such as holoprosencephaly (most common malformation of the mammalian forebrain), in which the typical separation of the two hemispheres is disrupted (the telencephalon does not divide into two sets), resulting in developmental defects of midline facial structures (such as cyclopia: single eye). 4  Yong Lu, Ph.D., Neural Development, PI note, M1 B&B D. Synaptogenesis & Circuit Formation Axonal growth To form functional circuits, axons of neurons grow in length, reach their targets in distance (pathfinding), and form synapses (synaptogenesis). The tip of an elongating axon expands as a growth cone with filopodia. The growth cone is a sensory-motor structure that recognizes and responds to guidance cues. Filopodia adheres to tissue substrates and directs the path of axonal extension. Axon growth is also guided by chemical and morphological cues. Growth cone guides the axon by transducing positive and negative cues into signals that regulate the cytoskeleton and thereby determine the course and rate of axon growth. Axons grow about 3-4 mm/day. The options for a developing axon: it can grow, turn, or stop. Pathway guidance cues act in diverse ways, by either promoting or inhibiting neurite growth. Growth cones encounter some molecules in the extracellular matrix, on cell surfaces, and still others of soluble form. After some “pioneer” axons reach their target, they serve as guides for subsequent axons, which find their way simply by following their predecessors. Axons reach distant targets in a series of discrete steps, making decisions at relatively frequent intervals along the way. Chemical cues play major roles in guiding the process. Molecular signals for axon pathfinding Chemoattractants: diffusible molecules that attract growth cones toward their targets, e.g. netrin (the first one that was discovered), which are secreted by ectodermal cells and are important guides for axons that must cross the midline (commissural axons), e.g. spinothalamic axons, majority of corticospinal axons, and half of the retinal axons. Chemorepellents: diffusible molecules that chase growth cones away, e.g. slit, semaphorin. Note that the determination of such a diffusible molecule depends on its action loci; some molecules can act as a chemoattractant in some region and as a chemorepellent in other regions. Other molecules important for growth: GAP-43 (growth associated protein) is important for axonal growth and guidance (and is up- regulated during regeneration). NoGo-A is an axon inhibitory factor found in CNS myelin. The example below shows how a growing axon can be “attracted” Bear et al., Neuroscience, 3rd and “repelled” by a chemoattractant netrin and a chemorepellent slit ed, Fig 23.11 5  Yong Lu, Ph.D., Neural Development, PI note, M1 B&B in the spinal cord. Cells in the ventral midline secret both netrin and slit (both are protein molecules), and there is a concentration gradient for both (high around the ventral midline, and lower in surrounding areas). Before the growing axon crosses the midline, the growth cone expresses netrin receptors (e.g. DCC, deleted in colorectal cancer) (and no slit receptor Robo is present), so the growth cone is attracted toward the midline and grows crossing the midline. After crossing the midline, Robo expression is up- regulated (genetically controlled). Binding of Robo with slit repels the axon and the axon grows away from the midline. Failure of proper function of attractant and repellent molecular cues can negatively impact proper development of parts of the NS and thus proper functioning. Clinical note: Robo (slit receptor) and horizontal gaze palsy Mutation in Robo3 affects the development of corticospinal tract, resulting in no crossing of the tract as well as the trochlear nerve (observed with Diffusion tensor imaging, DTI). The disorder is termed Horizontal gaze palsy, because patients with this disorder have limited ability to move their eyes horizontally. Patients also have difficulties in postural and motor control, because muscle activity is regulated by activity of the ipsilateral motor cortex (unlike in normal subjects, by contralateral and bilateral motor cortex). Synaptogenesis Most of knowledge about how synapses form comes from studies of the neuro-muscular junction (NMJ). Synapse formation involves three key events: Target recognition: formation of selective connections between the developing axon and its target. Growth cones of axons are guided to their appropriate target by combinations of signaling molecules in the extracellular space. Presynaptic differentiation: transformation of the axon’s growth cone into a nerve terminal, and establishment of transmitter release machinery. Elaboration of a postsynaptic apparatus in the target cell; a concentration of receptors accumulate at the future postsynaptic active site. Synapses in the CNS develop in similar ways to NMJ, but different molecules are used. E. Apoptosis: living or death During development there is an overproduction of neurons; probably 2-3 times more than are necessary for the adult; so half of the neurons born will die in normal development. Cell death is an active and necessary process in development. Factors essential for neuronal survival are provided by the target of the neuron; this is demonstrated by a simple experiment: removal of target of spinal cord motor 6  Yong Lu, Ph.D., Neural Development, PI note, M1 B&B neurons leads to increased cell death in the ventral horn of spinal cord). Target cells secret neurotrophic factors, which bind with receptors on axon terminals, internalized, and retrogradely transported to the cell bodies (recall retrograde axonal transport of growth factors). The first such factor discovered is named nerve growth factor (NGF). A large array of secreted factors have been found, among which neurotrophins are the best studied. Four major neurotrophins (NT) in the order of being discovered: NGF, BDNF (brain-derived neurotrophic factor), NT3, NT4/5. They are expressed in different tissues, and interact with two classes of receptors, tyrosine kinase (trk) A, B, C, and P75NTR; the binding can be specific or non-specific. While the survival of PNS neurons depends on neurotrophins, the role of neurotrophic factors in the CNS remains uncertain. Neurotrophins suppress an endogenous cell death program. Cell death occurs to those neurons that do not (or are not selected for) make appropriate connections in a circuit. In the absence of neurotrophins, the cell death program is activated, leading to a process characterized by four features: cell shrinkage, condensation of chromatin, cellular fragmentation, and phagocytosis of cellular remnants. This cell death, called apoptosis, is different from that resulting from injury or disease (necrosis) in that it occurs by active transcription of genes that turn on to cause degeneration. Trk receptor activation suppresses apoptosis, whereas activation of p75 receptors can lead to cell survival or cell death depending on the signaling pathway. Apoptosis may be involved in certain neurological disorders such as Parkinson’s disease and amyotrophic lateral sclerosis (ALS). Research on neurotrophic factor signaling and the biochemistry of cell death mechanisms is beginning to be applied to the search for treatment of neurodegenerative disorders. F. Synapse Rearrangement Following apoptosis, neural connections are reexamined to make sure that each target cell is innervated by the right number of axons, and each axon innervates the right number of target cells. Synaptic rearrangement is a general term; it includes processes such as synapse elimination, synapse increment, and synaptic segregation. E.g., polyneuronal innervation of immature muscle (one muscle cell/fiber innervated by multiple motor neuron axons). After maturation, one muscle fiber receives 7  Yong Lu, Ph.D., Neural Development, PI note, M1 B&B innervation of only one alpha motor axon. Note: one alpha motor neuron can still innervate and control multiple muscle fibers (motor unit). Synaptic rearrangement involves neurotrophins, as well as competition among axons from different sources for “ownership” of a neuron. The competition is thought to be modulated by patterns of electrical activity in the pre- and postsynaptic partners. Mechanism of experience-dependent changes: Hebb’s theorem (Hebbian Rule): Simultaneous activity of a presynaptic and postsynaptic neuron will strengthen the synaptic connection between them. In other words, “Cells that fire together, wire together.” Synapses with persistent correlated activity will be retained and/or strengthen, and processes from the presynaptic cell may sprout new branches. Correlated activity across presynaptic inputs will more strongly drive firing in the postsynaptic cell than uncorrelated activity across presynaptic inputs. Thus, correlated presynaptic inputs drive pre- and postsynaptic correlation. Synapses with persistent uncorrelated activity will vanish and/or weaken, and the presynaptic cell may lose branches. Uncorrelated presynaptic inputs will drive less firing and induce this effect. This is the cellular basis of learning and memory. It applies to long-term modifications of synaptic strength, including during development. Modification of circuits involves: changes in anatomical connectivity, and changes in synaptic strength. These changes can include differences in: numbers of synapses from each presynaptic neuron, and synaptic strength changes, including (but not limited to): changes at presynaptic terminals and responses at postsynaptic terminals such as number, type, and configuration of ion channels. IV. Postnatal brain development Molecular matching predominates during embryonic development (Chemospecificity hypothesis), and activity/experience modifies the circuits once they have been established. Some alterations in neurons and circuits occur developmentally. It has been suggested in patients with schizophrenia that there is a developmental neural disorganization such that some neurons don’t migrate and don’t grow properly. Postnatally, most changes occur in the cerebral cortex. About 70% of CNS neurons are in the cerebral cortex; cortical development differentiates humans from animals. Learning & behavioral demands & sensorimotor training increase dendritic extensions & synapses of cortical pyramidal cells. Some neurogenesis occurs postnatally. This occurs from neural stem cells in selected areas of the nervous system such as in hippocampus. The new cells may contribute to the hippocampus structure and function (involved in learning and memory). Drugs of abuse: opiates, ethanol, & nicotine, are known to decrease the number of new hippocampal neurons. Children have an astonishing capacity to recover lost function (due to damage) by increasing the structure (and function) of the remaining cortices, possibly because synaptic strength is not maximized at all synapses in their CNS. 8  Yong Lu, Ph.D., Neural Development, PI note, M1 B&B Myelination: myelination starts in the 2nd trimester, and continues after birth until adulthood. Clinical note: The development of the ability of movements in babies coincides with the development of myelination of the corticospinal tract. Infants display the Babinski sign due to incomplete myelination of the tract during normal development. Chiari malformation: if part of the skull containing the cerebellum is deformed during development, cerebellar structures (tonsils and/or vermis) may herniate through the foramen magnum and behind the upper cervical cord. In adulthood, this can result in headaches and cerebellar symptoms. Often associated with spinal cavity (e.g. syringomyelia), which results in loss of pain and temperature sensation of the involved body parts because the cavity is usually located in the central spinal cord destroying the white anterior commissure. There are four different types of Chiari malformation. Types I & II are more commonly seen than types III & IV. Type I: tonsils herniation, associated with syringomyelia (cavity in the cord, cervical level, resulting in “cape-like” loss of pain/temperature sensation of the upper limbs). Type II: tonsils and vermis herniation, associated with myelomeningocele (lumbosacral level). Dandy-Walker malformation: gene mutations (VLDLR: very low density lipoprotein receptor) or chromosomal abnormalities lead to agenesis of cerebellar vermis. Patients have enlarged 4th ventricle, which occupies a large volume of the posterior fossa. Associated with hydrocephalus (too much “water” in the brain). Associated with spina bifida and agenesis of the corpus callosum. Result in problems in movement (cerebellar symptoms; loss of coordination, balance, etc.). 9