Collapsin response mediator protein family or CRMP family consists of five intracellular

phosphoproteins A phosphoprotein is a protein that is posttranslationally modified by the attachment of either a single phosphate group, or a complex molecule such as 5'-phospho-DNA, through a phosphate group. The target amino acid is most often serine, threonin ...

( CRMP-1, CRMP-2, CRMP-3, CRMP4, CRMP5) of similar molecular size (60–66 kDa) and high (50–70%) amino acid sequence identity. CRMPs are predominantly expressed in the

nervous system In Biology, biology, the nervous system is the Complex system, highly complex part of an animal that coordinates its Behavior, actions and Sense, sensory information by transmitting action potential, signals to and from different parts of its ...

during development and play important roles in

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 ...

formation from neurites and in growth cone guidance and collapse through their interactions with

microtubules Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nm and have an inner diameter between 11 a ...

. Cleaved forms of CRMPs have also been linked to neuron degeneration after trauma induced injury. The modulation of CRMP-2 expression through various pharmaceuticals is a new and expanding area of research. By discovering chemicals that can either increase or decrease CRMP-2 expression, scientists can potentially reduce the effects of neurological diseases such as Alzheimer's disease and

Parkinson's disease Parkinson's disease (PD), or simply Parkinson's, is a long-term degenerative disorder of the central nervous system that mainly affects the motor system. The symptoms usually emerge slowly, and as the disease worsens, non-motor symptoms becom ...

History

Members of the CRMP family were discovered independently in different species by several groups working in parallel. Among the five members of the family, CRMP-2 was first identified in 1995. Group of researchers led by Goshima found out that CRMP-2 played a role in the transduction of the extracellular Semaphorin 3A (Sema3A), an inhibitory protein for axonal guidance in chick

dorsal root ganglion A dorsal root ganglion (or spinal ganglion; also known as a posterior root ganglion) is a cluster of neurons (a ganglion) in a dorsal root of a spinal nerve. The cell bodies of sensory neurons known as first-order neurons are located in the dor ...

(DRG). The protein was first named as CRMP-62 having a relative molecular mass of 62 kDa and later referred as CRMP-2. Concurrently, a 64 kDa protein named as TOAD-64 for Turned On After Division, was shown to increase significantly during the development of the cortex of the

brain The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head ( cephalization), usually near organs for special ...

. The cDNA sequence of TOAD-64 corresponded to that of rat CRMP-2. In 1996, mouse CRMP-4, often referred to as Ulip for Unc-33 like phosphoprotein, was discovered by Byk and colleagues, using a rabbit polyclonal antiserum which recognized a 64 kDa mouse brain specific phosphoprotein. In the same year, several other studies cloned CRMPs-1-4 in rat and dihydropyrimidinase (DHPase)

homologous Homology may refer to: Sciences Biology *Homology (biology), any characteristic of biological organisms that is derived from a common ancestor *Sequence homology, biological homology between DNA, RNA, or protein sequences * Homologous chrom ...

sequence of CRMPs-1, -2, and -4 in human fetal brain. Finally, in 2000, CRMP-5 was discovered using two-hybrid screenings of brain libraries or purification from a proteic complex. In following researches, CRMPs were studied as target antigens for autoantibodies in various autoimmune neurodegenerative disorders.

Structure

CRMP1-5 are between 564 and 572 amino acids and these proteins are found to be approximately 95% conserved between mouse and human. The protein sequence of CRMP1-4 is approximately 75%

with each other, while CRMP5 is only 50-51%

with each of the other CRMPs. Additionally, CRMPs are homologs of Unc-33 whose

mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, m ...

causes impaired ability to form neural circuits and uncoordinated mobility in ''

Caenorhabditis elegans ''Caenorhabditis elegans'' () is a free-living transparent nematode about 1 mm in length that lives in temperate soil environments. It is the type species of its genus. The name is a blend of the Greek ''caeno-'' (recent), ''rhabditis'' (r ...

''. CRMP1-4 genes are roughly 60%

with the tetramer liver dihydropyrimidinase (DHPase), and also possess a similar structure to members of the metal-dependent

amidohydrolases Amidohydrolases (or amidases) are a type of hydrolase that acts upon amide bonds. They are categorized under EC number EC 3.5.1 and 3.5.2. Examples include: * Beta-lactamase * Histone deacetylase * Urease The amidohydrolase superfamily is a lar ...

. However, the fact that CRMPs are not enzymatic reveals that they might lack the critical '' His'' residues that are present in amidohydrolase enzymes to allow them to bind metal atoms to their active site. Additionally, CRMPs can exist as homotetramers or as heterotetramers. The tetramers are positioned so that the active residues on the N-terminal are located on the outside of the complex. This allows CRMP to regulate various factors in the cytoplasm. Gel filtration analysis has shown that CRMP-5 and CRMP-1 form weaker homo-tetramers compared with CRMP-2, and that divalent cations, Ca2+ and Mg2+, destabilize oligomers of CRMP-5 and CRMP-1, but promote CRMP-2 oligomerization. The

C-terminus The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When the protein i ...

consists of 80 amino acids and is the site of

phosphorylation In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, ...

for various

kinases In biochemistry, a kinase () is an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates. This process is known as phosphorylation, where the high-energy ATP molecule donat ...

Expression

The expression of CRMPs is regulated throughout development of the

. In general, CRMPs are highly expressed in post-mitotic nerve cells since early

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...

nic life. In the developing

, each CRMP displays a distinct expression pattern both in time and space. For example, in the external granular layer (EGL), where

mitosis In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is maint ...

of cerebellar granular neuron occurs, CRMP-2 is highly expressed while CRMP-5 is never expressed. However, CRMP-2 and CRMP-5 are found to be co-expressed in post-mitotic granular neurons. CRMP expression is highest when

neurons 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. ...

and synaptic connections mature actively during the first postnatal week, suggesting CRMPs’ role in neuronal migration, differentiation and axonal growth. Indeed, CRMP-2 expression is induced by neuronal differentiation promoting factors such as noggin, chordin, GDNF, and FGF. In the adult

, CRMP expression is significantly downregulated and limited in areas associated with brain plasticity, neurogenesis, or regeneration. CRMP1 mRNA is mainly expressed in Purkinje cells of the

cerebellum The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cere ...

. Among the five members of the CRMP family, CRMP-2 is the most highly expressed in the adult brain, especially in post-mitotic

of the olfactory system,

, and

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, ...

. CRMP-3 mRNA is only expressed in the granular layer of the

, inferior olive, and dentate gyrus of the

. CRMP-4 is the least expressed protein of CRMP family and its expression is restricted to the olfactory bulb,

, and the internal granule layer (IGL) of the

. Lastly, CRMP-5 is expressed not only in post-mitotic

of the olfactory bulb, olfactory epithelium, and dentate gyrus of the

, but also in

peripheral A peripheral or peripheral device is an auxiliary device used to put information into and get information out of a computer. The term ''peripheral device'' refers to all hardware components that are attached to a computer and are controlled by th ...

nerve axons and sensory neurons. Other families of CRMP also appear in

tissues. Expression of CRMPs-1, -4, and -5 in the adult testis is detected only in the cell spermatid stage and CRMP-2 mRNA is found in lung tissue of the fetal mouse and adult human. The expression of CRMPs also can be found in the death or survival signaling of postmitotic neurons. Although CRMP is a cytosolic protein, significant amount of CRMP expression is detected as membrane associated at the leading edge of the growth cone lamellipodium and filopodia. Also, injury-induced CRMPs expression is found in sprouting fibers in both the central and

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 a ...

. CRMP-4 expression is promoted upon ischemic injury and is associated with neurons having intact morphology, suggesting that CRMP-4 provides a survival signal and may be involved in regeneration of neurons. Similarly, CRMP-2 has been suggested to participate in the survival and maintenance in postmitotic neurons as its over-expression accelerates nerve regeneration. However, CRMP-2 may also be involved in neuronal death as its expression is upregulated during the early stages of dopamine-induced neuronal apoptosis in cerebellar granule neurons.

Mechanism, Function and Regulation

Axonal formation in developing neuron

CRMP-2 plays a role in neuronal

polarity Polarity may refer to: Science * Electrical polarity, direction of electrical current * Polarity (mutual inductance), the relationship between components such as transformer windings * Polarity (projective geometry), in mathematics, a duality of o ...

. Extensions of early

neuron A neuron, neurone, or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous ...

s called lamellipodia form the early neurites. The

neurite A neurite or neuronal process refers to any projection from the cell body of a neuron. This projection can be either an axon or a dendrite. The term is frequently used when speaking of immature or developing neurons, especially of cells in cultur ...

s are indistinguishable between dendrites and the

during this stage. One of these

s eventually becomes the

and grows longer than the dendritic

s. CRMP-2 helps facilitate the rate of this axonal growth through its interactions with microtubules. CRMP-2 binds to and copolymerizes with tubulin heterodimers but does not bind as well to polymerized tubulin. This binding specificity promotes tubulin

polymerization In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many fo ...

in vitro ''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in biology and ...

. CRMP-2/ tubulin complexes are found in the distal part of the

and modulate microtubule dynamics by controlling the rate of microtubule assembly. CRMP-2 also contributes to the establishment of neuronal

by regulating polarized Numb-mediated

endocytosis Endocytosis is a cellular process in which substances are brought into the cell. The material to be internalized is surrounded by an area of cell membrane, which then buds off inside the cell to form a vesicle containing the ingested material. E ...

at the axonal growth cones. In both cases,

of CRMP-2 at Thr-555 by

Rho kinase Rho-associated protein kinase (ROCK) is a kinase belonging to the AGC (PKA/ PKG/PKC) family of serine-threonine specific protein kinases. It is involved mainly in regulating the shape and movement of cells by acting on the cytoskeleton. ROCKs ( ...

or at Thr-509, Thr-514 or Ser-518 by

GSK-3β Glycogen synthase kinase-3 beta, (GSK-3 beta), is an enzyme that in humans is encoded by the ''GSK3B'' gene. In mice, the enzyme is encoded by the Gsk3b gene. Abnormal regulation and expression of GSK-3 beta is associated with an increased susce ...

inactivates the protein by lowering binding affinity to tubulin and Numb.

Axonal growth cone guidance

In the developing nervous system, CRMPs’ involvement in

axonal guidance Axon guidance (also called axon pathfinding) is a subfield of neural development concerning the process by which neurons send out axons to reach their correct targets. Axons often follow very precise paths in the nervous system, and how they man ...

has been proposed by localization of CRMPs in neurites and axonal growth cones. CRMPs participate in two distinct transduction pathways inducing axonal growth cone collapse. Both pathways involve

Rho Rho (uppercase Ρ, lowercase ρ or ; el, ρο or el, ρω, label=none) is the 17th letter of the Greek alphabet. In the system of Greek numerals it has a value of 100. It is derived from Phoenician letter res . Its uppercase form uses the sa ...

family

GTPases GTPases are a large family of hydrolase enzymes that bind to the nucleotide guanosine triphosphate (GTP) and hydrolyze it to guanosine diphosphate (GDP). The GTP binding and hydrolysis takes place in the highly conserved P-loop "G domain", ...

RhoA Transforming protein RhoA, also known as Ras homolog family member A (RhoA), is a small GTPase protein in the Rho family of GTPases that in humans is encoded by the ''RHOA'' gene. While the effects of RhoA activity are not all well known, it i ...

and Rac1, in their signaling cascade. Rho family GTPases regulate the cytoskeletal reorganization of the growth cone and affect the growth cone motility. In Sema3A

signaling cascade A biochemical cascade, also known as a signaling cascade or signaling pathway, is a series of chemical reactions that occur within a biological cell when initiated by a stimulus. This stimulus, known as a first messenger, acts on a receptor that ...

, CRMP plays a role as intracellular messenger mediating repulsive signal. Sema3A initiates clustering of the receptor neuropilin 1 and plexin A1. While some of the other class of semaphorins directly bind to plexin receptors, Sema3A does not bind to plexin directly. Instead, it interacts with neuropilins as ligand-binding co-receptor for plexin and releases plexin-based signaling. The signal transduction pathway downstream of activated plexin receptor is mediated by CRMPs. In response to Sema3A signaling cascade, CRMPs which exist as a heterotetramer in the cytosol bind to the cytosolic domain of PlexA and its conformation changes. Further, CRMPs are phosphorylated by Cdk5, GSK3B, and Fes, a tyrosine protein kinase. Especially,

of CRMP-1 and CRMP-2 are essential for Sema3A-regulated

. In the presence of CRMP-2, the signal can induce alterations of

Rac RAC or Rac may refer to: Organizations * Radio Amateurs of Canada * RATCH-Australia Corporation, electricity generator * Refugee Action Collective (Victoria), Melbourne, Australia * Religious Action Center of Reform Judaism, US * Rent-A-Center, ...

-dependent pathway, which modulates the actin filament assembly in the growth cone. In the absence of Sema3A, the interaction between CRMP tetramer and PlexA is blocked. Phospholipase D2 (PLD-2) which is localized in the growth cone and is involved in

actin Actin is a protein family, family of Globular protein, globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in myofibril, muscle fibrils. It is found in essentially all Eukaryote, eukaryotic cel ...

cytoskeleton The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is comp ...

rearrangement, can be inhibited by CRMP-2 and its inhibition results in actin depolymerization and possibly affects axonal growth cone collapse. In the presence of CRMP-2, the signal can induce alterations of

-dependent pathway, which modulates the

filament assembly in the growth cone. CRMP-2 is also involved in another growth cone collapse signal induced by extracellular

lysophosphatidic acid Lysophosphatidic acid (LPA) is a phospholipid derivative that can act as a lipid signaling, signaling molecule. Function LPA acts as a potent mitogen due to its activation of three high-affinity G-protein-coupled receptors called LPAR1, LPAR2, a ...

(LPA). A signal through seven-transmembrane receptor activates an intracellular pathway,

and the downstream of RhoA, Rho-kinase subsequently phosphorylates CRMP-2 on Threonine-555 (Thr555). In DRG neurons, CRMP-2 is phosphorylated by

in LPA signaling but not in Sema3A signaling, revealing the presence of both Rho kinase-dependent and Rho kinase-independent pathways for the growth cone collapse. In

pathway, CRMP-1 interacts with Rho-kinase and modulates RhoA signaling. CRMP-2 can be regulated post-translationally by O-GluNAc (β-N-acetylglucosamine linked to hydroxyls of serine or threonine) as the modification blocks CRMP-2 from being phosphorylated.

Trauma induced degeneration

Cleaved CRMP products play a considerable role in the degeneration of axons as a result of trauma inflicted on 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 p ...

(CNS). As a result of trauma induced on the CNS, glutamate activates NMDA receptors leading to an influx of calcium that activates the calcium-dependent protease calpain. It has been shown that activated calpain proteolytically cleaves CRMP-3, creating a cleavage product of CRMP that interacts with vital cytosolic and nuclear molecules to bring about neurodegeneration. The structure of this cleaved form of CRMP has not been determined yet, making it difficult to understand the protein-protein interactions that occur and why these forms are able to initiate neurodegeneration after CNS injury. Additionally, calpain inhibitors (ALLN) are shown to have prevented the CRMP‐3 cleavage and therefore no axonal degeneration or neuronal death, further suggesting that calpain targets CRMP-3 for cleavage during glutamate-induced neuronal death. Ca2+/calmodulin-dependent protein kinase II (CaMK II) is also activated by calcium influx through NMDA receptors, and is another possible activator of CRMP-3. CRMP-3 is not the only CRMP involved in neuronal degeneration brought upon by trauma and cerebral ischemia, as all CRMPs are in fact targeted for cleavage to help promote degeneration.

List of CRMPs (and associated knockout phenotypes and derived functions)

Clinical significance

The expression of CRMPs is altered in neurodegenerative diseases and these proteins likely play an essential role in the pathogenesis of disorders in the nervous system, including Alzheimer's disease,

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 wi ...

, and many others. One pharmaceutical that is relatively effective in targeting CRMP-2 to reduce the results of a neurodegenerative disease is lacosamide. Lacosamide is used in combination with other types of medications to control various types of seizures, especially

epilepsy Epilepsy is a group of non-communicable neurological disorders characterized by recurrent epileptic seizures. Epileptic seizures can vary from brief and nearly undetectable periods to long periods of vigorous shaking due to abnormal electrical ...

. One of the ways lacosamide does this is by modulating CRMP-2, thus inducing neuroprotective effects and decreasing the epileptic effects in people with

. CRMP-2 phosphorylated at Thr-509, Ser-518, and Ser-522 has been connected to the degenerating neuritis in Alzheimer's disease. Studies suggest that glycogen synthase kinase-3β (GSK-3β) and cyclin-dependent protein kinase 5 ( Cdk5) are highly expressed in Alzheimer's disease and are some of the

protein kinases A protein kinase is a kinase which selectively modifies other proteins by covalently adding phosphates to them (phosphorylation) as opposed to kinases which modify lipids, carbohydrates, or other molecules. Phosphorylation usually results in a fun ...

responsible for inactivating CRMP-2 in Alzheimer's disease. This inactivation of CRMP-2 in people with Alzheimer's disease promotes the expression of neurofibrillary tangles and plaque neurites which are consistent with people with this disease. CRMP-2 is also related to

bipolar disorder Bipolar disorder, previously known as manic depression, is a mental disorder characterized by periods of depression and periods of abnormally elevated mood that last from days to weeks each. If the elevated mood is severe or associated with ...

and

, likely as a result of the phosphorylation of CRMP-2 by

References

{{reflist Molecular neuroscience Protein families