Collapsin response mediator protein family

Members of the CRMP family were discovered independently in different species by several groups working in parallel.[4][6] 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 (DRG).[6] 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 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.[6] In the same year, several other studies cloned CRMPs-1-4 in rat and dihydropyrimidinase (DHPase) homologous sequence of CRMPs-1, -2, and -4 in human fetal brain.[6] Finally, in 2000, CRMP-5 was discovered using two-hybrid screenings of brain libraries or purification from a proteic complex.[6] In following researches, CRMPs were studied as target antigens for autoantibodies in various autoimmune neurodegenerative disorders.[6]

Crystal structure of CRMP-1

CRMP1-5 are between 564 and 572 amino acids and these proteins are found to be approximately 95% conserved between mouse and human.[6] The protein sequence of CRMP1-4 is approximately 75% homologous with each other, while CRMP5 is only 50-51% homologous with each of the other CRMPs.[4] Additionally, CRMPs are homologs of Unc-33 whose mutation causes impaired ability to form neural circuits and uncoordinated mobility in Caenorhabditis elegans.[7] CRMP1-4 genes are roughly 60% homologous with the tetramer liver dihydropyrimidinase (DHPase), and also possess a similar structure to members of the metal-dependent amidohydrolases. 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.[4]

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.[8] The C-terminus consists of 80 amino acids and is the site of phosphorylation for various kinases.[4]

The expression of CRMPs is regulated throughout development of the nervous system. In general, CRMPs are highly expressed in post-mitotic nerve cells since early embryonic life. In the developing nervous system, each CRMP displays a distinct expression pattern both in time and space. For example, in the external granular layer (EGL), where mitosis 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.[6] CRMP expression is highest when neurons and synaptic connections mature actively during the first postnatal week, suggesting CRMPs’ role in neuronal migration, differentiation and axonal growth.[4][6] Indeed, CRMP-2 expression is induced by neuronal differentiation promoting factors such as noggin, chordin, GDNF, and FGF.[4]

In the adult nervous system, 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. Among the five members of the CRMP family, CRMP-2 is the most highly expressed in the adult brain, especially in post-mitotic neurons of the olfactory system, cerebellum, and hippocampus. CRMP-3 mRNA is only expressed in the granular layer of the cerebellum, inferior olive, and dentate gyrus of the hippocampus. CRMP-4 is the least expressed protein of CRMP family and its expression is restricted to the olfactory bulb, hippocampus, and the internal granule layer (IGL) of the cerebellum. Lastly, CRMP-5 is expressed not only in post-mitotic neurons of the olfactory bulb, olfactory epithelium, and dentate gyrus of the hippocampus, but also in peripheral nerve axons and sensory neurons. Other families of CRMP also appear in peripheral 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.[6]

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.[4] 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.[7]

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, Parkinson's disease, schizophrenia, 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. One of the ways lacosamide does this is by modulating CRMP-2, thus inducing neuroprotective effects and decreasing the epileptic effects in people with epilepsy.[10]

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 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.[11][12] CRMP-2 is also related to bipolar disorder and schizophrenia, likely as a result of the phosphorylation of CRMP-2 by GSK-3β.[12]