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Synaptic pruning, a phase in the
development of the nervous system The development of the nervous system, or neural development (neurodevelopment), refers to the processes that generate, shape, and reshape the nervous system of animals, from the earliest stages of embryonic development to adulthood. The fiel ...
, is the process of
synapse In the nervous system, a synapse is a structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or to the target effector cell. Synapses are essential to the transmission of nervous impulses from ...
elimination that occurs between early childhood and the onset of puberty in many
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s, including
human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
s. Pruning starts near the time of birth and continues into the late-20s. During pruning, both the
axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action po ...
and
dendrite Dendrites (from Greek δένδρον ''déndron'', "tree"), also dendrons, are branched protoplasmic extensions of a nerve cell that propagate the electrochemical stimulation received from other neural cells to the cell body, or soma, of the n ...
decay and die off. It was traditionally considered to be complete by the time of
sexual maturation Sexual maturity is the capability of an organism to reproduce. In humans it might be considered synonymous with adulthood, but here puberty is the name for the process of biological sexual maturation, while adulthood is based on cultural definiti ...
, but this was discounted by MRI studies. The infant brain will increase in size by a factor of up to 5 by adulthood, reaching a final size of approximately 86 (± 8) billion
neuron A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...
s. Two factors contribute to this growth: the growth of synaptic connections between neurons and the
myelination Myelin is a lipid-rich material that surrounds nerve cell axons (the nervous system's "wires") to insulate them and increase the rate at which electrical impulses (called action potentials) are passed along the axon. The myelinated axon can be l ...
of
nerve fibers A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...
; the total number of neurons, however, remains the same. After adolescence, the volume of the synaptic connections decreases again due to synaptic pruning. Pruning is influenced by environmental factors and is widely thought to represent
learning Learning is the process of acquiring new understanding, knowledge, behaviors, skills, value (personal and cultural), values, attitudes, and preferences. The ability to learn is possessed by humans, animals, and some machine learning, machines ...
.


Variations


Regulatory pruning

At birth, the neurons in the visual and motor cortices have connections to the
superior colliculus In neuroanatomy, the superior colliculus () is a structure lying on the roof of the mammalian midbrain. In non-mammalian vertebrates, the homologous structure is known as the optic tectum, or optic lobe. The adjective form ''tectal'' is commonly ...
,
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spi ...
, and
pons The pons (from Latin , "bridge") is part of the brainstem that in humans and other bipeds lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum. The pons is also called the pons Varolii ("bridge of Va ...
. The neurons in each cortex are selectively pruned, leaving connections that are made with the functionally appropriate processing centers. Therefore, the neurons in the
visual cortex The visual cortex of the brain is the area of the cerebral cortex that processes visual information. It is located in the occipital lobe. Sensory input originating from the eyes travels through the lateral geniculate nucleus in the thalamus and ...
prune the synapses with neurons in the spinal cord, and the
motor cortex The motor cortex is the region of the cerebral cortex believed to be involved in the planning, control, and execution of voluntary movements. The motor cortex is an area of the frontal lobe located in the posterior precentral gyrus immediately a ...
severs connections with the superior colliculus. This variation of pruning is known as large-scaled stereotyped axon pruning. Neurons send long axon branches to appropriate and inappropriate target areas, and the inappropriate connections are eventually pruned away. Regressive events refine the abundance of connections, seen in
neurogenesis Neurogenesis is the process by which nervous system cells, the neurons, are produced by neural stem cells (NSCs). It occurs in all species of animals except the porifera (sponges) and placozoans. Types of NSCs include neuroepithelial cells (NECs) ...
, to create a specific and mature circuitry.
Apoptosis Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes incl ...
and pruning are the two main methods of severing the undesired connections. In apoptosis, the neuron is killed and all connections associated with the neuron are also eliminated. In contrast, the neuron does not die in pruning, but requires the retraction of
axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action po ...
s from synaptic connections that are not functionally appropriate. It is believed that the purpose of synaptic pruning is to remove unnecessary neuronal structures from the brain; as the human brain develops, the need to understand more complex structures becomes much more pertinent, and simpler associations formed at childhood are thought to be replaced by complex structures. Despite the fact it has several connotations with regulation of cognitive childhood development, pruning is thought to be a process of removing neurons which may have become damaged or degraded in order to further improve the "networking" capacity of a particular area of the brain. Furthermore, it has been stipulated that the mechanism not only works in regard to development and reparation, but also as a means of continually maintaining more efficient brain function by removing neurons by their synaptic efficiency.


Pruning in the maturing brain

The pruning that is associated with learning is known as small-scale axon terminal arbor pruning. Axons extend short axon terminal arbors toward neurons within a target area. Certain terminal arbors are pruned by competition. The selection of the pruned terminal arbors follow the "use it or lose it" principle seen in
synaptic plasticity In neuroscience, synaptic plasticity is the ability of synapses to strengthen or weaken over time, in response to increases or decreases in their activity. Since memories are postulated to be represented by vastly interconnected neural circuit ...
. This means synapses that are frequently used have strong connections while the rarely used synapses are eliminated. Examples seen in vertebrate include pruning of axon terminals in the
neuromuscular junction A neuromuscular junction (or myoneural junction) is a chemical synapse between a motor neuron and a muscle fiber. It allows the motor neuron to transmit a signal to the muscle fiber, causing muscle contraction. Muscles require innervation to ...
in the
peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system of bilateral animals, with the other part being the central nervous system (CNS). The PNS consists of nerves and ganglia, which lie outside the brain ...
and the pruning of
climbing fiber Climbing fibers are the name given to a series of neuronal projections from the inferior olivary nucleus located in the medulla oblongata. These axons pass through the pons and enter the cerebellum via the inferior cerebellar peduncle whe ...
inputs to the cerebellum in the
central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all par ...
. In terms of humans, synaptic pruning has been observed through the inference of differences in the estimated numbers of
glial cell Glia, also called glial cells (gliocytes) or neuroglia, are non-neuronal cells in the central nervous system (brain and spinal cord) and the peripheral nervous system that do not produce electrical impulses. They maintain homeostasis, form mye ...
s and neurons between children and adults, which differs greatly in the mediodorsal thalamic nucleus. In a study conducted in 2007 by
Oxford University Oxford () is a city in England. It is the county town and only city of Oxfordshire. In 2020, its population was estimated at 151,584. It is north-west of London, south-east of Birmingham and north-east of Bristol. The city is home to the ...
, researchers compared 8 newborn human brains with those of 8 adults using estimates based upon size and evidence gathered from stereological
fractionation Fractionation is a separation process in which a certain quantity of a mixture (of gases, solids, liquids, enzymes, or isotopes, or a suspension) is divided during a phase transition, into a number of smaller quantities (fractions) in which the ...
. They showed that, on average, estimates of adult neuron populations were 41% lower than those of the newborns in the region they measured, the mediodorsal thalamic nucleus. However, in terms of glial cells, adults had far larger estimates than those in newborns; 36.3 million on average in adult brains, compared to 10.6 million in the newborn samples. The structure of the brain is thought to change when degeneration and deafferentation occur in postnatal situations, although these phenomena have not been observed in some studies. In the case of development, neurons which are in the process of loss via programmed cell death are unlikely to be re-used, but rather replaced by new neuronal structures or synaptic structures, and have been found to occur alongside the structural change in the sub-cortical
gray matter Grey matter is a major component of the central nervous system, consisting of neuronal cell bodies, neuropil (dendrites and unmyelinated axons), glial cells (astrocytes and oligodendrocytes), synapses, and capillaries. Grey matter is distinguis ...
. Synaptic pruning is classified separately from the regressive events seen during older ages. While developmental pruning is experience dependent, the deteriorating connections that are synonymous with old age are not. The stereotyped pruning can be compared to the process of chiseling and molding of stone into a statue. Once the statue is complete, the weather will begin to erode the statue and this represents the experience-independent deletion of connections.


Forgetting problems with learning through pruning

All attempts to construct
artificial intelligence Artificial intelligence (AI) is intelligence—perceiving, synthesizing, and inferring information—demonstrated by machines, as opposed to intelligence displayed by animals and humans. Example tasks in which this is done include speech re ...
systems that learn by pruning connections that are disused have the problem that every time they learn something new, they forget everything they learned before. Since biological brains follow the same laws of physics as artificial intelligences, as all physical objects do, these researchers argue that if biological brains learned by pruning they would face the same catastrophic forgetting issues. This is pointed out as an especially severe problem if the learning is supposed to be part of a developmental process since retention of older knowledge is necessary for developmental types of learning, and as such it is argued that synaptic pruning cannot be a mechanism of mental development. It is argued that developmental types of learning must use other mechanisms that do not rely on synaptic pruning.


Energy saving for reproduction and discontinuous differences

One theory of why many brains are synaptically pruned when a human or other primate grows up is that maintenance of synapses consume
nutrient A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...
s which may be needed elsewhere in the body during growth and sexual maturation. This theory presupposes no mental function of synaptic pruning. The empirical observation that human brains fall into two distinct categories, one that reduces synaptic density by about 41% while growing up and another synaptically
neotenic Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing of the physiological, or somatic, development of an organism, typically an animal. Neoteny is found in modern humans compared ...
type in which there is very little to no reduction of synaptic density, but no continuum between them, is explainable by this theory as an adaptation to physiologies with different nutritional needs in which one type needs to free up nutrients to get through puberty while the other can mature sexually by other redirections of nutrients that do not involve reducing the brain's consumption of nutrients. Citing that most of the nutrient costs in the brain are in maintaining the brain cells and their synapses, rather than the firing itself, this theory explains the observation that some brains appear to continue pruning years after sexual maturation as a result of some brains having more robust synapses, allowing them to take years of neglect before the synaptic spines finally disintegrate. Another hypothesis that can explain the discontinuity is that of limited functional genetic space restricted by the fact that most of the human genome needs to lack sequence-specific functions to avoid too many deleterious mutations, predicting that evolution proceeds by a few of the mutations happening to have large effects while most mutations do not have any effects at all.P. Michael Conn (2011)."Handbook of Models for Human Aging"


Mechanisms

The three models explaining synaptic pruning are axon degeneration, axon retraction, and axon shedding. In all cases, the
synapses In the nervous system, a synapse is a structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or to the target effector cell. Synapses are essential to the transmission of nervous impulses from ...
are formed by a transient
axon terminal Axon terminals (also called synaptic boutons, terminal boutons, or end-feet) are distal terminations of the telodendria (branches) of an axon. An axon, also called a nerve fiber, is a long, slender projection of a nerve cell, or neuron, that condu ...
, and synapse elimination is caused by the axon pruning. Each model offers a different method in which the axon is removed to delete the synapse. In small-scale axon arbor pruning, neural activity is thought to be an important regulator, but the molecular mechanism remains unclear.
Hormones A hormone (from the Ancient Greek, Greek participle , "setting in motion") is a class of cell signaling, signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and beh ...
and
trophic factor A growth factor is a naturally occurring substance capable of stimulating cell proliferation, wound healing, and occasionally cellular differentiation. Usually it is a secreted protein or a steroid hormone. Growth factors are important for reg ...
s are thought to be the main extrinsic factors regulating large-scale stereotyped axon pruning.


Axon degeneration

In ''
Drosophila ''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species ...
'', there are extensive changes made to the nervous system during
metamorphosis Metamorphosis is a biological process by which an animal physically develops including birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation. Some inse ...
. Metamorphosis is triggered by
ecdysone Ecdysone is a prohormone of the major insect molting hormone 20-hydroxyecdysone, which is secreted from the prothoracic glands. It is of steroidal structure. Insect molting hormones (ecdysone and its homologues) are generally called ecdysteroids. ...
, and during this period, extensive pruning and reorganization of the neural network occurs. Therefore, it is theorized that pruning in ''Drosophila'' is triggered by the activation of ecdysone receptors.
Denervation Denervation is any loss of nerve supply regardless of the cause. If the nerves lost to denervation are part of the neuronal communication to a specific function in the body then altered or a loss of physiological functioning can occur. Denervation ...
studies at the neuromuscular junction of vertebrates have shown that the axon removal mechanism closely resembles
Wallerian degeneration Wallerian degeneration is an active process of degeneration that results when a nerve fiber is cut or crushed and the part of the axon distal to the injury (i.e. farther from the neuron's cell body) degenerates. A related process of dying back o ...
. However, the global and simultaneous pruning seen in ''Drosophilia'' differs from the mammalian nervous system pruning, which occurs locally and over multiple stages of development.


Axon retraction

Axon branches retract in a
distal Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
to
proximal Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
manner. The axonal contents that are retracted are thought to be recycled to other parts of the axon. The biological mechanism with which axonal pruning occurs still remains unclear for the mammalian central nervous system. However, pruning has been associated with guidance molecules in mice. Guidance molecules serve to control axon pathfinding through repulsion, and also initiate pruning of exuberant synaptic connections.
Semaphorin Semaphorins are a class of secreted and membrane proteins that were originally identified as axonal growth cone guidance molecules. They primarily act as short-range inhibitory signals and signal through multimeric receptor complexes. Semaphorins ...
ligands In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electro ...
and the receptors neuropilins and
plexins A plexin is a protein which acts as a receptor for semaphorin family signaling proteins. It is classically known for its expression on the surface of axon growth cones and involvement in signal transduction to steer axon growth away from the sou ...
are used to induce retraction of the axons to initiate hippocampo-septal and infrapyramidal bundle (IPB) pruning. Stereotyped pruning of the hippocampal projections have been found to be significantly impaired in mice that have a Plexin-A3 defect. Specifically, axons that are connected to a transient target will retract once the Plexin-A3 receptors are activated by class 3 semaphorin ligands. In IPB, the expression of mRNA for Sema3F is present in the
hippocampus The hippocampus (via Latin from Greek , 'seahorse') is a major component of the brain of humans and other vertebrates. Humans and other mammals have two hippocampi, one in each side of the brain. The hippocampus is part of the limbic system, a ...
prenatally, lost postnatally and returns in the stratum oriens. Coincidentally, onset IPB pruning occurs around the same time. In the case of the hippocampal-septal projections, expression of mRNA for Sema3A was followed by the initiation of pruning after 3 days. This suggests that pruning is triggered once the ligand reaches threshold protein levels within a few days after detectable
mRNA In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of Protein biosynthesis, synthesizing a protein. mRNA is ...
expression. Pruning of axons along the visual
corticospinal tract The corticospinal tract is a white matter motor pathway starting at the cerebral cortex that terminates on lower motor neurons and interneurons in the spinal cord, controlling movements of the limbs and trunk. There are more than one million neur ...
(CST) is defective in neuropilin-2 mutants and plexin-A3 and plexin-A4 double mutant mice. Sema3F is also expressed in the dorsal spinal cord during the pruning process. There is no motor CST pruning defect observed in these mutants. Stereotyped pruning has also been observed in the tailoring of overextended axon branches from the
retinotopy Retinotopy (from Greek τόπος, place) is the mapping of visual input from the retina to neurons, particularly those neurons within the visual stream. For clarity, 'retinotopy' can be replaced with 'retinal mapping', and 'retinotopic' with 'r ...
formation.
Ephrin Ephrins (also known as ephrin ligands or Eph family receptor interacting proteins) are a family of proteins that serve as the ligands of the Eph receptor. Eph receptors in turn compose the largest known subfamily of receptor protein-tyrosine kinas ...
and the
ephrin receptor Eph receptors (Ephs, after erythropoietin-producing human hepatocellular receptors) are a group of receptors that are activated in response to binding with Eph receptor-interacting proteins (Ephrins). Ephs form the largest known subfamily of rec ...
s, Eph, have been found to regulate and direct retinal axon branches. Forward signaling between ephrin-A and EphA, along the
anterior Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
- posterior axis, has been found to inhibit retinal axon branch formation posterior to a terminal zone. The forward signaling also promotes pruning of the axons that have reached into the terminal zone. However, it remains unclear whether the retraction mechanism seen in IPB pruning is applied in retinal axons. Reverse signaling between ephrin-B proteins and their Eph
receptor tyrosine kinases Receptor tyrosine kinases (RTKs) are the high-affinity cell surface receptors for many polypeptide growth factors, cytokines, and hormones. Of the 90 unique tyrosine kinase genes identified in the human genome, 58 encode receptor tyrosine kinase ...
have been found to initiate the retraction mechanism in the IPB. Ephrin-B3 is observed to transduce tyrosine phosphorylation-dependent reverse signals into hippocampal axons that trigger pruning of excessive IPB fibers. The proposed pathway involves EphB being expressed on the surface of target cells that results in tyrosine phosphorylation of ephrin-B3. Ensuing binding of ephrin-B3 to the cytoplasmic adaptor protein, Grb4, leads to the recruitment and binding of Dock180 and
p21 activated kinases p21 activated kinases (PAKs) are members of a family of enzymes. They serve as targets for the small GTP binding proteins CDC42 and Rac and have been implicated in a wide range of biological activities. Members include: * PAK1, regulating cell ...
(PAK). The binding of Dock180 increases Rac-GTP levels, and PAK mediates the downstream signaling of active Rac that leads to the retraction of the axon and eventual pruning.


Axon shedding

Time-lapse imaging of retreating axons in
neuromuscular junctions A neuromuscular junction (or myoneural junction) is a chemical synapse between a motor neuron and a muscle fiber. It allows the motor neuron to transmit a signal to the muscle fiber, causing muscle contraction. Muscles require innervation to ...
of mice have shown axonal shedding as a possible mechanism of pruning. The retreating axon moved in a
distal Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
to
proximal Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
order and resembled retraction. However, there were many cases in which remnants were shed as the axons were retracting. The remnants, named axosomes, contained the same organelles seen in the bulbs attached to the end of axons and were commonly found around the proximity of the bulbs. This indicates that axosomes are derived from the bulbs. Furthermore, axosomes did not have electron-dense
cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. The ...
s or disrupted
mitochondria A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...
indicating that they were not formed through Wallerian degeneration.


Potential role in schizophrenia

Synaptic pruning has been suggested to have a role in the pathology of neurodevelopmental disorders such as
schizophrenia Schizophrenia is a mental disorder characterized by continuous or relapsing episodes of psychosis. Major symptoms include hallucinations (typically hearing voices), delusions, and disorganized thinking. Other symptoms include social withdra ...
, as well as in
autism spectrum disorder The autism spectrum, often referred to as just autism or in the context of a professional diagnosis autism spectrum disorder (ASD) or autism spectrum condition (ASC), is a neurodevelopmental disorder, neurodevelopmental condition (or conditions) ...
. ''''
Microglia Microglia are a type of neuroglia (glial cell) located throughout the brain and spinal cord. Microglia account for about 7% of cells found within the brain. As the resident macrophage cells, they act as the first and main form of active immune de ...
have been implicated in synaptic pruning, as they have roles in both the immune response as
macrophage Macrophages (abbreviated as M φ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell of the immune system that engulfs and digests pathogens, such as cancer cel ...
s as well as in neuronal upkeep and
synaptic plasticity In neuroscience, synaptic plasticity is the ability of synapses to strengthen or weaken over time, in response to increases or decreases in their activity. Since memories are postulated to be represented by vastly interconnected neural circuit ...
in the CNS during fetal development, early postnatal development, and adolescence, in which they engulf unneeded or redundant synapses via
phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...
. '''' Microglial synapse engulfment and uptake has been specifically observed to be upregulated in the isolated
synaptosome A synaptosome is an isolated synaptic terminal from a neuron. Synaptosomes are obtained by mild homogenization of nervous tissue under isotonic conditions and subsequent fractionation using differential and density gradient centrifugation. Liquid s ...
s of male patients with schizophrenia compared to healthy controls, suggesting upregulated microglia-induced synaptic pruning in these individuals. Microglia-mediated synaptic pruning has also been observed to be upregulated during late adolescence and early adulthood, which could also account for the age of onset for schizophrenia often being reported around this time in development (late teens to early 20s for men, and mid-to-late 20s for women) The drug minocycline, a semisynthetic brain-penetrant tetracycline antibiotic, has been found to somewhat reverse these changes made to patient synaptosomes by downregulating synaptic pruning. Genes in the
Complement Component 4 Complement component 4 (C4), in humans, is a protein involved in the intricate complement system, originating from the human leukocyte antigen (HLA) system. It serves a number of critical functions in immunity, tolerance, and autoimmunity with ...
(C4) locus of the
major histocompatibility complex The major histocompatibility complex (MHC) is a large locus on vertebrate DNA containing a set of closely linked polymorphic genes that code for cell surface proteins essential for the adaptive immune system. These cell surface proteins are calle ...
(MHC), which encode for complement factors, have also been tied to schizophrenia risk through gene linkage studies. The fact that some of these complement factors are involved in signaling during synaptic pruning also seems to suggest that schizophrenia risk may be linked to synaptic pruning. Specifically, complement factors C1q and C3 have been found to have a role in microglia-mediated synaptic pruning. '''' Carriers of C4 risk variants have also been found to be tied to this kind of synapse overpruning in microglia. The proposed mechanism for this interaction is increased complement factor C3 deposition onto synaptosomes as a consequence of increased C4A expression in these risk variant carriers.


See also

*
Neuroplasticity Neuroplasticity, also known as neural plasticity, or brain plasticity, is the ability of Neural circuit, neural networks in the brain to change through growth and reorganization. It is when the brain is rewired to function in some way that diffe ...


References

{{reflist Developmental neuroscience