A comparative study of soluble calcium-dependent proteolytic activity in brain

Two novel homozygous mutations of CAPN1 in Chinese patients with hereditary spastic paraplegia and literatures review

BACKGROUND

Hereditary spastic paraplegias (HSP) are of great clinical and genetic heterogeneity. According to the clinical features, HSP can be divided into pure or complicated subtypes which combined with other neurological symptoms including cerebellar ataxia. Up to date, 78 loci or genes have been implicated in HSP. CAPN1 was a novel gene detected recently for spastic paraplegia 76 (SPG76).

METHODS

Patients referred to our clinic with spastic or spastic-ataxic gait were collected. Genetic testing of the probands were performed by target sequencing of a panel containing over 4000 known virulence genes. And the candidate mutations were further confirmed by polymerase chain reaction (PCR) and Sanger sequencing. The clinical materials of these patients were demonstrated retrospectively.

RESULTS

Two Chinese patients, both from consanguineous families, each carried a novel homozygous mutation of CAPN1, p.R48X and p.R339X. The male proband presented pure HSP subtype while the female proband presented complicated HSP symptoms with cerebellar ataxia. We then reviewed all the literatures of HSP patients carrying CAPN1 mutations and summarized the molecular spectrum and clinical characteristics of CAPN1-related SPG76.

CONCLUSION

These two SPG76 patients carrying CAPN1 mutations were the first reported in China. By reviewing the clinical manifestations of SPG76 patients, we validated the "spastic-ataxia" phenotype and emphasized the association between spasticity and ataxia, indicating the importance of CAPN1 screening in HSP patients.

Defects in the CAPN1 Gene Result in Alterations in Cerebellar Development and Cerebellar Ataxia in Mice and Humans

A CAPN1 missense mutation in Parson Russell Terrier dogs is associated with spinocerebellar ataxia. We now report that homozygous or heterozygous CAPN1-null mutations in humans result in cerebellar ataxia and limb spasticity in four independent pedigrees. Calpain-1 knockout (KO) mice also exhibit a mild form of ataxia due to abnormal cerebellar development, including enhanced neuronal apoptosis, decreased number of cerebellar granule cells, and altered synaptic transmission. Enhanced apoptosis is due to absence of calpain-1-mediated cleavage of PH domain and leucine-rich repeat protein phosphatase 1 (PHLPP1), which results in inhibition of the Akt pro-survival pathway in developing granule cells. Injection of neonatal mice with the indirect Akt activator, bisperoxovanadium, or crossing calpain-1 KO mice with PHLPP1 KO mice prevented increased postnatal cerebellar granule cell apoptosis and restored granule cell density and motor coordination in adult mice. Thus, mutations in CAPN1 are an additional cause of ataxia in mammals, including humans.

Lysosomes and brain aging in mammals

Hypotheses about the factors controlling the rate of brain aging are usually derived from 1) correlates of maximum life span across mammals or 2) investigations into the causes of age-related neuropathologies in humans. With regard to the former, the strong correlation between metabolic rate and longevity prompted a variety of free radical hypotheses of aging. There is also evidence that brain size affects life span independently of body metabolism rates. The second approach has led to a diverse array of pathogenic mechanisms and, importantly for the development of general hypotheses, the discovery of animal analogues. The present paper discusses the possibility that age-associated lysosomal dysfunction constitutes a generalized mammalian phenomenon that accounts for specific features of the aged human brain. Immunocytochemical studies using rats and dogs have identified lysosomal changes that begin early in adulthood and are most pronounced in brain areas known to be particularly vulnerable to age-related pathogenesis in humans. Experimentally induced lysosomal dysfunction in cultured brain slices from rats and mutant mice triggers a wide array of changes associated with the aged human brain, including meganeurites and intraneuronal tangles. Finally, there is evidence that at least some forms of proteolysis decrease with increasing brain size across the mammals. The above observations lead to the suggestion that the expansion of neuronal arborizations that occurred in conjunction with increases in brain size secondarily slowed both neuronal metabolism and protein turnover. These events could have served to reduce the rate at which lysosomes (and other organelles) fail.

Transient brain ischaemia provokes Ca2+, PIP2 and calpain responses prior to delayed neuronal death in monkeys

To clarify the mechanism of postischaemic delayed cornu Ammonis (CA)-1 neuronal death, we studied correlations among calpain activation and its subcellular localization, the immunoreactivity of phosphatidylinositol 4,5-bisphosphate (PIP2) and Ca2+ mobilization in the monkey hippocampus by two independent experimental approaches: in vivo transient brain ischaemia and in vitro hypoxia-hypoglycaemia of hippocampal acute slices. The CA-1 sector undergoing 20 min of ischaemia in vivo showed microscopically a small number of neuronal deaths on day 1 and almost global neuronal loss on day 5 after ischaemia. Immediately after ischaemia, CA-1 neurons ultrastructurally showed vacuolation and/or disruption of the lysosomes. Western blotting using antibodies against inactivated or activated mu-calpain demonstrated mu-calpain activation specifically in the CA-1 sector immediately after ischaemia. This finding was confirmed in the perikarya of CA-1 neurons by immunohistochemistry. CA-1 neurons on day 1 showed sustained activation of mu-calpain, and increased immunostaining for inactivated and activated forms of mu- and m-calpains and for PIP2. Activated mu-calpain and PIP2 were found to be localized at the vacuolated lysosomal membrane or endoplasmic reticulum and mitochondrial membrane respectively, by immunoelectron microscopy. Calcium imaging data using hippocampal acute slices showed that hypoxia-hypoglycaemia in vitro provoked intense Ca2+ mobilization with increased PIP2 immunostaining specifically in CA-1 neurons. These data suggest that transient brain ischaemia increases intracellular Ca2+ and PIP2 breakdown, which will activate calpain proteolytic activity. Therefore, we suggest that activated calpain at the lysosomal membrane, with the possible release of biodegrading enzyme, will cause postischaemic CA-1 neuronal death.

The localization of mcalpain in myelin: immunocytochemical evidence in different areas of rat brain and nerves

A major part of brain mcalpain activity has been found associated with myelin, but its presence in the myelin sheath has not been clearly demonstrated by microscopic (morphological) means. Using myelin mcalpain antisera the localization of mcalpain has been investigated in tissue of rat CNS and PNS by immunohistochemical methods. These experiments also have been carried out by double labeling studies using antibodies to myelin basic protein (MBP) and neurofilament protein (NFP). Our results indicate calpain/MBP immunoreactivity in the myelin sheath surrounding the axon while NFP antibody stained inside the axon in spinal cord; pons, cerebellum, trigeminal nerve, and sciatic nerve. Patches of light immunoreactivity of calpain were also seen in the axonal cytoplasm. The calpain immunostaining of myelin was similar to that of MBP staining indicating the presence of calpain in myelin. This finding supports the view that calpain is a constituent of myelin, may be involved in the normal turnover of myelin proteins. In pathological situations such as in demyelinating and other brain degenerative diseases, myelin may be autodigestive.

Calcium-activated proteolysis of neurofilament proteins in goldfish Mauthner axons

We have examined the proteolytic breakdown of neurofilament proteins (NFPs) in isolated Mauthner axoplasm (M-axoplasm). Documentation of proteolytic breakdown of NFPs in M-axoplasm is important because NFPs are not degraded in distal segments of severed Mauthner axons (M-axons) maintained in vivo for up to 62 days at 20 degrees C. By incubating M-axoplasm with 2 mM calcium in vitro, we have demonstrated that M-axoplasm contains an endogenous calcium-activated neutral protease that degrades NFPs. This calcium-activated proteolysis of M-axoplasm NFPs produced novel bands on silver-stained gels. These novel bands were presumed to be NFP breakdown products because they reacted with antibodies to the alpha-intermediate filament antigen (anti-IFA) on immunoblots from these gels. Incubations of M-axoplasm with 2 mM calcium plus exogenous calpain produced novel bands similar to those observed for M-axoplasm incubated with 2 mM calcium. Incubations of M-axoplasm with 2mM calcium plus calpain inhibitors did not produce these novel bands. These in vitro data indicate that M-axoplasm contains calpain that degrades NFPs and produces novel bands similar to those observed from distal segments of severed M-axons maintained in vivo longer than 62 days postseverance. Factors that affect the activity of calpain or affect the ability of calpain to degrade NFPs could account for the delayed degradation of NFPs in distal segments of severed M-axons maintained in vivo.

Distribution of calcium-activated neutral protease inhibitor in the central nervous system of the rat

The ubiquitous existence of calcium-activated neutral protease (CANP, calpain), an enzyme whose activity is regulated by calcium ions and a specific endogenous CANP inhibitor (calpastatin), is well known. Although there has been much investigation concerning the distribution and role of CANP, investigations of the distribution of the CANP inhibitor using immunohistochemical techniques are rare. We made antiserum against a 40K fragment of cDNA corresponding to two C-terminal repeats of rat liver CANP inhibitor expressed in Escherichia coli. Using this antiserum, we examined the distribution of CANP inhibitor in the rat central nervous system by the ABC technique and compared it with the distribution of CANP. Neurons and glias were stained, with the cytosol stained diffusely and the cell membranes stained clearly and strongly. Axons and myelin were stained faintly, but nuclei and vessels were not stained. The distribution of CANP inhibitor was thus found to be similar to that of CANP.

Complex interactions between polyamines and calpain-mediated proteolysis in rat brain

Polyamine synthesis is induced by various extracellular signals, and it is widely held that this biochemical response participates in cell growth and differentiation. Certain of the triggers for synthesis in brain tissues also increase the breakdown of high-molecular-weight structural proteins, apparently by activating calcium-dependent proteases (calpains). The present experiments tested the possibility that calpain activity is modulated by polyamines. Spermine, spermidine, and putrescine all increased calcium-dependent proteolysis of [14C]casein by soluble fractions of rat brain. The order of potency was spermine greater than spermidine greater than putrescine, with apparent affinities of 30, 300, and 6,000 microM, respectively. Each of the three polyamines at physiological concentrations also potentiated the calcium-dependent breakdown of two endogenous high-molecular-weight structural proteins known to be substrates of calpain, in both supernatant and membrane fractions. The thiol protease inhibitor leupeptin, a known calpain inhibitor, also inhibited calcium-dependent proteolysis in the presence and absence of polyamines. The polyamines did not increase the activity of purified calpain I or calpain II determined with either [14C]casein or purified spectrin as the substrate, nor did they interfere with the inhibitory effects of calpastatin, an endogenous inhibitor of calpain. However, polyamines potentiated the stimulation of endogenous but not purified calpain activity produced by an endogenous calpain activator. These results suggest a role for polyamines in protein degradation as well as protein synthesis.

Distribution of calcium-activated protease calpain in the rat brain

Calpain is a calcium-activated neutral protease that degrades a number of cytoskeletal proteins. It may participate in the maintenance of the cytoskeleton and in the rapid turnover of structural proteins associated with synaptic plasticity. Calpain may also be involved in the neurodegeneration that accompanies aging and age-related diseases. To aid in the interpretation of disease-related alterations in staining patterns, the present study examined calpain's normal distribution in the mammalian brain and spinal cord. A monoclonal antibody was employed with the avidin-biotin-peroxidase immunocytochemical technique on samples of rat tissue. Glia (astrocytes, microglia) and virtually all neurons were immunopositive, although neuronal processes exhibited varying staining patterns. The axonal staining pattern depended upon either the origin or destination of the process: those axons remaining within the brain (e.g., corpus callosum) were only lightly immunoreactive, whereas spinal cord and peripheral axons (trigeminal nerve) were more darkly labeled. The architecture of the dendritic tree determined the dendritic staining pattern: neurons with prominent apical and basal dendritic trees (e.g., pyramidal cells) were immunolabeled along their entire extent; labeling of multipolar cells (e.g., hilar cells of dentate gyrus) was limited to the proximal dendrites. The ubiquitous distribution of calpain argues against a primary role for the enzyme in the regional pattern of neuronal death seen in Alzheimer's disease. An alteration in the concentration, localization, or inhibition of the enzyme could, however, lead to the abnormal accumulations of cytoskeletal elements seen with the disease.

Calpain and calpastatin activity in the optic pathway

The levels of the neutral proteolytic enzymes calpains and their endogenous inhibitor calpastatin were determined in the retina and in the retrobulbar optic pathway in the albino rabbit. The highest level of calpains was observed in the optic nerve with decreasing levels in the optic tract and superior colliculus. The level of calpastatin in the retina was very low compared to that in the optic nerve and tract and other parts of the nervous system.

The calcium-dependent proteolytic system calpain-calpastatin in Drosophila melanogaster

Ca2+-dependent proteolytic activity was detected at pH 7.5 in head extracts of the fruit fly Drosophila melanogaster. This activity was abolished by iodoacetate, but was unaffected by phenylmethanesulphonyl fluoride. These properties resemble those of the Ca2+-dependent thiol-proteinase calpain. The activity appeared at Mr 280,000 on Sepharose CL-6B gel chromatography. DEAE-cellulose chromatography revealed two activity peaks, with elution positions corresponding to vertebrate calpains I and II. The fly head enzymes were inhibited by a heat-stable and trypsin-sensitive component of the fly head extract, which also inhibited calpains from rat kidney. The inhibitor emerged from Sepharose CL-6B columns at Mr 310,000 and from DEAE-cellulose at a position corresponding to the protein inhibitor calpastatin from other sources. It is concluded that Drosophila heads comprise the Ca2+-dependent calpain-calpastatin proteolytic system.

Excitatory amino acids activate calpain I and induce structural protein breakdown in vivo

Neuronal activity regulates the catabolism of specific structural proteins in adult mammalian brain. Pharmacological stimulation of rat hippocampal neurons by systemic or intraventricular administration of the excitatory amino acids kainate or N-methyl-D-aspartate induces selective loss of brain spectrin and the microtubule-associated protein MAP2, as determined by quantitative immunoblotting, but not of actin, the high molecular weight neurofilament polypeptide, or glial fibrillary acidic protein. The spectrin decrease occurs primarily by enhanced proteolysis, as levels of the major breakdown products of the alpha-subunit increase more than 7-fold. This proteolysis may occur from activation of the calcium-dependent neutral protease calpain I. The immunopeptide maps produced by alpha-spectrin degradation, selective loss of spectrin and MAP2, and decrease in calpain I levels are all consistent with calpain I activation accompanied by autoproteolysis. We propose that calcium influx and calpain I activation provide a mechanism by which neuronal activity regulates the degradation of specific neuronal structural proteins and may thereby modify neuronal morphology.

Proteolysis of tubulin and microtubule-associated proteins 1 and 2 by calpain I and II. Difference in sensitivity of assembled and disassembled microtubules

Calpain I and II (EC 3.4.22.17) are Ca2+-activated neutral thiol-proteases. Isolated brain tubulin and microtubule-associated proteins were found to be good substrates for proteolytic degradation by brain calpain I and II. The assembly of microtubules was totally inhibited when the calpains were allowed to act on microtubule proteins initially, and a complete disassembly was found after addition of calpain I to assembled microtubules. The high-molecular weight microtubule-associated proteins were degraded within a few minutes following incubation with calpain as shown by SDS-polyacrylamide gel electrophoresis and electron microscopy. When calpain was added to pre-formed microtubules, either in the presence or in the absence of microtubule-associated proteins, the proteolysis was significantly reduced. When tubulin was pre-assembled by taxol, the formation of proteolytic fragments was decreased indicating that assembly alters the availability of tubulin sites for proteolytic cleavage by calpain. Digested tubulin spontaneously formed aberrant polymers. No considerable change of apparent net charge was seen, thus indicating that calpain cleaves off fragments containing neutral amino acid residues and/or that the fragments of tubulin remain associated as an entity with the same charge as native tubulin. The results suggest that the calpains act as irreversible microtubule regulators.

Electrically coupled but chemically isolated synapses: dendritic spines and calcium in a rule for synaptic modification

An influential model of learning assumes synaptic enhancement occurs when there is pre- and post-synaptic conjunction of neuronal activity, as proposed by Hebb (1949) and studied in the form of long-term potentiation (LTP). There is evidence that LTP has a post-synaptic locus of control and is triggered by an elevation of intracellular calcium ion concentration, [Ca2+]i. Since synapses which undergo LTP are usually situated on dendritic spines, three effects of spine morphology on this system should be considered: (i) synapses on spines are chemically isolated by the barrier to Ca2+ diffusion due to the spine neck dimensions; (ii) the resistance of the spine neck permits a given synaptic current to bring about greater depolarization (of the spine head membrane) than the same current into a dendrite; while (iii) the spine neck resistance does not significantly attenuate current flow (in the dendrite to spine direction) because of the relatively high impedance of the spine head, and this permits electrical coupling via the dendritic tree. The specificity of LTP to activated synapses on depolarized cells has recently been attributed to special properties of the receptor-linked channel specifically activated by N-methyl-D-aspartate (NMDA). This admits calcium and other ions only when there is both depolarization and receptor activation. However, consideration of point (ii) suggests that, for spines with high resistance necks, the current through a synapse on the spine head will cause sufficient depolarization to unblock the NMDA channel. Thus, the properties of the NMDA channel do not account for the requirement for conjunction of pre- and post-synaptic activity, if these channels are located on the spine head. This suggests that additional mechanisms are required to explain why it is necessary to depolarize the post-synaptic cell in order to induce LTP. As an alternative, it is postulated that there exist voltage-sensitive calcium channels (VSCCs) on the spine head membrane, of a type which require greater membrane depolarization for activation. To generate the greater depolarization required, both pre- and post-synaptic activation would be necessary. If so, the role of dendritic or somatically located NMDA channels may be to "prime" neurons for LTP by enchancing voltage-dependent responses. A corollary is that spine resistance may regulate the threshold number of synapses required to produce LTP. It is predicted that, on spines with very high neck resistance (say, greater than 600 M omega), synaptic current alone may produce sufficient depolarization to activate VSCCs.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of the main body cations on the peptidase(s) activity against pyroGlu6[125I-Tyr8]SP6-11 in the nuclear and synaptosomal fraction of the cortex and hippocampus of rat brain

1. Peptidase(s) activity of nuclear and synaptosomal fraction from cortex and hippocampus of rat brain against pyroGlu6[125I-Tyr8]SP6-11 was evaluated in different concentration of Ca2+, Mg2+, K+ and Na+ in about "isotonic" conditions. 2. The effects of studied ions on the peptidase activities forming N-terminal and C-terminal fragments are different especially in synaptosomes of both areas. 3. The differences of ionic requirements for N- and C-forming activities are particularly relevant for Ca2+ at the cortex and K+ at the hippocampus. 4. Ca2+ activate forming of N-terminal fragments in the nuclear fraction whereas inhibit it in synaptosomes from both areas. 5. The ionic requirements for C-terminal fragments' formation in synaptosomes of both areas are contradictory.

Subcellular compartmentalization of calcium-dependent and calcium-independent neutral proteases in brain

In the present experiments, we studied the subcellular distribution of three types of extralysosomal, neutral proteolytic activities in rat telencephalon: (1) nonthiol proteases (NTP), (2) thiol proteases (TP), and (3) calcium-activated thiol proteases (calpains I and II). Subcellular fractionation was performed by using conventional differential and sucrose-gradient centrifugation techniques. The only significant proteolytic activity detected in crude homogenates could be assigned to calpain II, the high-threshold calcium-activated protease. Within the primary fractions prepared from the homogenates, the highest levels of calpain II were found in S3, or the soluble cytoplasmic fraction. Significant activity of the enzyme was also present in P2, the crude mitochondrial/synaptosomal fraction. In contrast, the specific activity of calpain I was greatest in P2 with somewhat lesser enzymatic activity in P1 and S3. Most of the calpain I in P2 was recovered after differential centrifugation through sucrose gradients and lysis of the resultant subfractions. In marked contrast, only a small percentage of the calpain II activity was recovered in the gradient bands. In all, calpain II appears to be predominantly localized in the soluble cytoplasmic compartment while the greatest concentrations of calpain I are found in the soluble components of small glial and neuronal processes (pinched off during homogenization) that constitute the P2 fraction. The highest specific activity of the calcium-independent proteases was obtained in P3, a fraction essentially devoid of calpain, with a secondary peak in P2. Subfractionation of P2 revealed that calcium-independent TP in P2 was associated with mitochondria while the calcium-independent NTP was more uniformly distributed across myelin, synaptosomes, and mitochondria.(ABSTRACT TRUNCATED AT 250 WORDS)

Low levels of calpain activity in Chiroptera brain: implications for mechanisms of aging

Calcium-dependent neutral proteases ("calpains") have been implicated in degenerative processes in muscles and neurons, suggesting that they might also play a role in age-related brain pathologies and perhaps in brain aging itself. Because Chiroptera exhibit an unusual maximum life span, relative to other mammalian orders, we investigated the activity of these enzymes in the brain of two species of bats. As in other mammals, brain calpain degrades many proteins associated with the cell cytoskeleton. However, enzyme activity is 5-7 fold lower in bat's brain than in a similar-sized mammal, such as the mouse. Moreover, the maximal life span of bats predicted from the equation relating calpain activity and maximal life span across a wide range of mammals is close to the observed values. These results strengthen the hypothesis that calpain activity is somehow linked to the rate at which brains age.