Localization of prosaposin in rat cochlea

Molecular differences between neonatal and adult stria vascularis from organotypic explants and transcriptomics

Summary

The stria vascularis (SV), part of the blood-labyrinth barrier, is an essential component of the inner ear that regulates the ionic environment required for hearing. SV degeneration disrupts cochlear homeostasis, leading to irreversible hearing loss, yet a comprehensive understanding of the SV, and consequently therapeutic availability for SV degeneration, is lacking. We developed a whole-tissue explant model from neonatal and adult mice to create a robust platform for SV research. We validated our model by demonstrating that the proliferative behaviour of the SV in vitro mimics SV in vivo, providing a representative model and advancing high-throughput SV research. We also provided evidence for pharmacological intervention in our system by investigating the role of Wnt/β-catenin signaling in SV proliferation. Finally, we performed single-cell RNA sequencing from in vivo neonatal and adult mouse SV and revealed key genes and pathways that may play a role in SV proliferation and maintenance. Together, our results contribute new insights into investigating biological solutions for SV-associated hearing loss.

Significance

Hearing loss impairs our ability to communicate with people and interact with our environment. This can lead to social isolation, depression, cognitive deficits, and dementia. Inner ear degeneration is a primary cause of hearing loss, and our study provides an in depth look at one of the major sites of inner ear degeneration: the stria vascularis. The stria vascularis and associated blood-labyrinth barrier maintain the functional integrity of the auditory system, yet it is relatively understudied. By developing a new in vitro model for the young and adult stria vascularis and using single cell RNA sequencing, our study provides a novel approach to studying this tissue, contributing new insights and widespread implications for auditory neuroscience and regenerative medicine.

Highlights

- We established an organotypic explant system of the neonatal and adult stria vascularis with an intact blood-labyrinth barrier. - Proliferation of the stria vascularis decreases with age in vitro , modelling its proliferative behaviour in vivo . - Pharmacological studies using our in vitro SV model open possibilities for testing injury paradigms and therapeutic interventions. - Inhibition of Wnt signalling decreases proliferation in neonatal stria vascularis.- We identified key genes and transcription factors unique to developing and mature SV cell types using single cell RNA sequencing.

Expression patterns of prosaposin and neurotransmitter-related molecules in the chick paratympanic organ

The paratympanic organ (PTO) is a small sense organ in the middle ear of birds that contains hair cells similar to those found in vestibuloauditory organs and receives afferent fibers from the geniculate ganglion. To consider the histochemical similarities between the PTO and vestibular hair cells, we examined the expression patterns of representative molecules in vestibular hair cells, including prosaposin, G protein-coupled receptor (GPR) 37 and GPR37L1 as prosaposin receptors, vesicular glutamate transporter (vGluT) 2 and vGluT3, nicotinic acetylcholine receptor subunit α9 (nAChRα9), and glutamic acid decarboxylase (GAD) 65 and GAD67, in the postnatal day 0 chick PTO and geniculate ganglion by in situ hybridization. Prosaposin mRNA was observed in PTO hair cells, supporting cells, and geniculate ganglion cells. vGluT3 mRNA was observed in PTO hair cells, whereas vGluT2 was observed in a small number of ganglion cells. nAChRα9 mRNA was observed in a small number of PTO hair cells. The results suggest that the histochemical character of PTO hair cells is more similar to that of vestibular hair cells than that of auditory hair cells in chicks.

Expression patterns of prosaposin and its receptors, G protein-coupled receptor (GPR) 37 and GPR37L1, in the mouse olfactory organ

Prosaposin is a glycoprotein conserved widely in vertebrates, because it is a precursor for saposins that are required for normal lysosomal function and thus for autophagy, and acts as a neurotrophic factor. Most tetrapods possess two kinds of olfactory neuroepithelia, namely, the olfactory epithelium (OE) and the vomeronasal epithelium (VNE). This study examined the expression patterns of prosaposin and its candidate receptors, G protein-coupled receptor (GPR) 37 and GPR37L1, in mouse OE and VNE by immunofluorescence and in situ hybridization. Prosaposin immunoreactivity was observed in the olfactory receptor neurons, vomeronasal receptor neurons, Bowman's gland (BG), and Jacobson's gland (JG). Prosaposin expression was mainly observed in mature neurons. Prosaposin mRNA expression was observed not only in these cells but also in the apical region of the VNE. GPR37 and GPR37L1 immunoreactivities were found only in the BG and/or the JG. Prosaposin was suggested to secrete and facilitate the autophagic activities of the neurons and modulate the mucus secretion in mouse olfactory organ.

Expression of prosaposin and its G protein-coupled receptor (GPR) 37 in mouse cochlear and vestibular nuclei

Prosaposin is a precursor of lysosomal hydrolases activator proteins, saposins, and also acts as a secretory protein that is not processed into saposins. Prosaposin elicits neurotrophic function via G protein-coupled receptor (GPR) 37, and prosaposin deficiency causes abnormal vestibuloauditory end-organ development. In this study, immunohistochemistry was used to examine prosaposin and GPR37 expression patterns in the mouse cochlear and vestibular nuclei. Prosaposin immunoreactivity was observed in neurons and glial cells in both nuclei. GPR37 immunoreactivity was observed in only some neurons, and its immunoreactivity in the vestibular nucleus was weaker than that in the cochlear nucleus. This study suggests a possibility that prosaposin deficiency affects not only the end-organs but also the first center of the vestibuloauditory system.

Prosaposin, a neurotrophic factor, protects neurons against kainic acid-induced neurotoxicity

Prosaposin (PS) is the precursor of four sphingolipid activator proteins, saposin A-D. PS is both a precursor protein and a neuroprotective factor, and is up-regulated in response to excitotoxicity induced by kainic acid (KA), a glutamate analogue. Excess glutamate release induces neuropathological disorders such as ischemia and seizure. Our group's research revealed that PS immunoreactivity (IR) increased significantly in the hippocampal and cortical neurons on day 3 after KA injection, and high PS levels were maintained even after 3 weeks. The increase in PS, but not saposins, as detected by immunoblotting, suggests that the increase in PS-IR after KA injection was not caused by an increase in saposins acting as lysosomal enzymes after neuronal damage but, rather, by an increase in PS as a neurotrophic factor to improve neuronal survival. An 18-mer peptide (PS18) derived from the PS neurotrophic region significantly protected hippocampal neurons against KA-induced destruction. Furthermore, parvalbumin-positive GABAergic inhibitory interneurons and their axons exhibited intense PS expression. These results suggest that axonally transported PS protects damaged hippocampal pyramidal neurons from KA-induced neurotoxicity. Further in vitro studies that include the transfection of the PS gene will help with clarifying the mechanisms underlying the transport and secretion of PS.

Prosaposin in the rat oviductal epithelial cells

Prosaposin (PSAP) has two forms: a precursor and a secreted form. The secreted form has neurotrophic, myelinotrophic, and myotrophic properties. The precursor form is a precursor protein of saposins A-D. Although the distribution of PSAP in male reproductive organs is well known, its distribution in female reproductive organs, especially in the oviduct, is unclear. Immunoblots and immunohistochemistry of oviducts showed that oviductal tissues contain PSAP proteins, and a significant increase in PSAP was observed in the estrus-metestrus phase compared to the diestrus-proestrus phase in the ampulla. To identify PSAP trafficking in cells, double-immunostaining was performed with antibodies against PSAP in combination with sortilin, mannose 6 phosphate receptor (M6PR), or low-density lipoprotein receptor-related protein 1 (LRP1). PSAP and sortilin double-positive reactions were observed near the nuclei, as well as in the apical portion of microvillous epithelial cells, whereas these reactions were only observed near the nuclei of ciliated epithelial cells. PSAP and M6PR double-positive reactions were observed near the nuclei of microvillous and ciliated epithelial cells. PSAP and M6PR double-positive reactions were also observed in the apical portion of microvillous epithelial cells. PSAP and LRP1 double-positive reactions were observed in the plasma membrane and apical portion of both microvillous and ciliated epithelial cells. Immunoelectron staining revealed PSAP immunoreactive small vesicles with exocytotic features at the apical portion of microvillous epithelial cells. These findings suggest that PSAP is present in the oviductal epithelium and has a pivotal role during pregnancy in providing an optimal environment for gametes and/or sperm in the ampulla.

Hypoxia-ischemia alters distribution of lysosomal proteins in rat cortex and hippocampus

Neuronal excitotoxicity induced by glutamatergic receptor overstimulation contributes to brain damage. Recent studies have shown that lysosomal membrane permeabilization (LMP) is involved in ischemia-associated neuronal death. In this study we evaluated the effect of neonatal hypoxia-ischemia (HI), as a model of excitotoxicity, on the lysosomal integrity throughout the distribution of the lysosomal proteins cathepsin D and prosaposin. Rat pups (7 days old) of the Wistar Kyoto strain were submitted to HI and they were euthanized 4 days after treatment and the cerebral cortex (Cx) and hippocampus (HIP) were processed for immunohistochemistry or immunoblotting. Treatment induced an increase of gliosis and also a redistribution of both prosaposin and cathepsin D (as intermediate and mature forms), into the cytosol of the HIP and Cx. In addition, HI induced a decrease of LAMP-1 in the membranous fraction and the appearance of a reactive band to anti-LAMP-1 in the cytosolic fraction, suggesting a cleavage of this protein. From these results, we propose that the abnormal release of Cat D and PSAP to the cytosol is triggered as a result of LAMP-1 cleavage in HI animals, which leads to cell damage. This could be a common mechanism in pathological conditions that compromises neuronal survival and brain function.

Summary: Hypoxia ischemia (HI) induces an increase of gliosis and redistribution of prosaposin and cathepsin D into the cytosol of rat hippocampus. This could be triggered by LAMP-1 cleavage in HI.

Interneurons secrete prosaposin, a neurotrophic factor, to attenuate kainic acid-induced neurotoxicity

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PS increased mainly in the axons of PV positive interneurons after kainic acid (KA) injection.

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Electron microscopy revealed PS containing vesicles in PV positive axons.

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PS is secreted with secretogranin from synapses.

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The increased PS in the interneurons was due to increases in PS + 0, as in the choroid plexus.

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Interneurons produce and secrete intact PS around the hippocampal pyramidal neurons to protect them from KA neurotoxicity.

Prosaposin (PS) is a secretory neurotrophic factor, as well as a regulator of lysosomal enzymes. We previously reported the up-regulation of PS and the possibility of its axonal transport by GABAergic interneurons after exocitotoxicity induced by kainic acid (KA), a glutamate analog. In the present study, we performed double immunostaining with PS and three calcium binding protein markers: parvalbumin (PV), calbindin, and calretinin, for the subpopulation of GABAergic interneurons, and clarified that the increased PS around the hippocampal pyramidal neurons after KA injection existed mainly in the axons of PV positive interneurons. Electron microscopy revealed PS containing vesicles in the PV positive axon. Double immunostaining with PS and secretogranin or synapsin suggested that PS is secreted with secretogranin from synapses. Based on the results from in situ hybridization with two alternative splicing forms of PS mRNA, the increase of PS in the interneurons was due to the increase of PS + 0 (mRNA without 9-base insertion) as in the choroid plexus, but not PS + 9 (mRNA with 9-base insertion). These results were similar to those from the choroid plexus, which secretes an intact form PS + 0 to the cerebrospinal fluid. Neurons, especially PV positive GABAergic interneurons, produce and secrete the intact form of PS around hippocampal pyramidal neurons to protect them against KA neurotoxicity.

Role of saposin C and D in auditory and vestibular function

OBJECTIVES/HYPOTHESIS

Saposins are small proteins derived from a precursor protein, prosaposin. Each of the four saposins (A-D) is necessary for the activity of lysosomal glycosphingolipid hydrolases. Individual saposin mutations lead to lysosomal storage diseases, some of which are associated with hearing loss. Here we evaluate the effects of the loss of saposins C and D on auditory and vestibular function in transgenic mice.

METHODS

Transgenic mice with either loss of saposin C function or a combined loss of saposin C + D function were studied. Light microscopy and immunofluorescence were used to evaluate histologic and morphologic changes in the auditory and vestibular organs. Acoustic brainstem response thresholds and distortion product otoacoustic emissions were used to study the auditory phenotype.

RESULTS

A null mutation of saposin C did not result in any identifiable histologic changes or loss of hearing through postnatal day 55. Combined losses of saposins C and D similarly did not result in any changes in organ of Corti histology or loss of hearing. However, inclusions within the vestibular end organs was noted, consistent with afferent and efferent neuronal sprouting, although to a much milder degree than seen in the previously studied prosaposin knockout mouse.

CONCLUSIONS

Loss of saposin C and D function, although causing mild phenotypic changes in the vestibular end organs, otherwise results in minimal functional impairment and no changes in the auditory system. It is more likely that the auditory and vestibular effects of the loss of prosaposin are mediated through the actions of saposin A and/or B.

LEVEL OF EVIDENCE

NA.

A prosaposin-derived Peptide alleviates kainic Acid-induced brain injury

Four sphingolipid activator proteins (i.e., saposins A–D) are synthesized from a single precursor protein, prosaposin (PS), which exerts exogenous neurotrophic effects in vivo and in vitro. Kainic acid (KA) injection in rodents is a good model in which to study neurotrophic factor elevation; PS and its mRNA are increased in neurons and the choroid plexus in this animal model. An 18-mer peptide (LSELIINNATEELLIKGL; PS18) derived from the PS neurotrophic region prevents neuronal damage after ischemia, and PS18 is a potent candidate molecule for use in alleviating ischemia-induced learning disabilities and neuronal loss. KA is a glutamate analog that stimulates excitatory neurotransmitter release and induces ischemia-like neuronal degeneration; it has been used to define mechanisms involved in neurodegeneration and neuroprotection. In the present study, we demonstrate that a subcutaneous injection of 0.2 and 2.0 mg/kg PS18 significantly improved behavioral deficits of Wistar rats (n = 6 per group), and enhanced the survival of hippocampal and cortical neurons against neurotoxicity induced by 12 mg/kg KA compared with control animals. PS18 significantly protected hippocampal synapses against KA-induced destruction. To evaluate the extent of PS18- and KA-induced effects in these hippocampal regions, we performed histological evaluations using semithin sections stained with toluidine blue, as well as ordinal sections stained with hematoxylin and eosin. We revealed a distinctive feature of KA-induced brain injury, which reportedly mimics ischemia, but affects a much wider area than ischemia-induced injury: KA induced neuronal degeneration not only in the CA1 region, where neurons degenerate following ischemia, but also in the CA2, CA3, and CA4 hippocampal regions.

Prosaposin overexpression following kainic acid-induced neurotoxicity

Because excessive glutamate release is believed to play a pivotal role in numerous neuropathological disorders, such as ischemia or seizure, we aimed to investigate whether intrinsic prosaposin (PS), a neuroprotective factor when supplied exogenously in vivo or in vitro, is up-regulated after the excitotoxicity induced by kainic acid (KA), a glutamate analog. In the present study, PS immunoreactivity and its mRNA expression in the hippocampal and cortical neurons showed significant increases on day 3 after KA injection, and high PS levels were maintained even after 3 weeks. The increase in PS, but not saposins, detected by immunoblot analysis suggests that the increase in PS-like immunoreactivity after KA injection was not due to an increase in saposins as lysosomal enzymes after neuronal damage, but rather to an increase in PS as a neurotrophic factor to improve neuronal survival. Furthermore, several neurons with slender nuclei inside/outside of the pyramidal layer showed more intense PS mRNA expression than other pyramidal neurons. Based on the results from double immunostaining using anti-PS and anti-GABA antibodies, these neurons were shown to be GABAergic interneurons in the extra- and intra-pyramidal layers. In the cerebral cortex, several large neurons in the V layer showed very intense PS mRNA expression 3 days after KA injection. The choroid plexus showed intense PS mRNA expression even in the normal rat, and the intensity increased significantly after KA injection. The present study indicates that inhibitory interneurons as well as stimulated hippocampal pyramidal and cortical neurons synthesize PS for neuronal survival, and the choroid plexus is highly activated to synthesize PS, which may prevent neurons from excitotoxic neuronal damage. To the best of our knowledge, this is the first study that demonstrates axonal transport and increased production of neurotrophic factor PS after KA injection.

Decrease in prosaposin in the Dystrophic mdx mouse brain

BACKGROUND

Duchenne muscular dystrophy caused by a mutation in the X-linked dystrophin gene induces metabolic and structural disorders in the brain. A lack of dystrophin in brain structures is involved in impaired cognitive function. Prosaposin (PS), a neurotrophic factor, is abundant in the choroid plexus and various brain regions. We investigated whether PS serves as a link between dystrophin loss and gross and/or ultrastructural brain abnormalities.

METHODOLOGY/PRINCIPAL FINDINGS

The distribution of PS in the brains of juvenile and adult mdx mice was investigated by immunochemistry, Western blotting, and in situ hybridization. Immunochemistry revealed lower levels of PS in the cytoplasm of neurons of the cerebral cortex, hippocampus, cerebellum, and choroid plexus in mdx mice. Western blotting confirmed that PS levels were lower in these brain regions in both juveniles and adults. Even with low PS production in the choroids plexus, there was no significant PS decrease in cerebrospinal fluid (CSF). In situ hybridization revealed that the primary form of PS mRNA in both normal and mdx mice was Pro+9, a secretory-type PS, and the hybridization signals for Pro+9 in the above-mentioned brain regions were weaker in mdx mice than in normal mice. We also investigated mitogen-activated protein kinase signalling. Stronger activation of ERK1/2 was observed in mdx mice, ERK1/2 activity was positively correlated with PS activity, and exogenous PS18 stimulated both p-ERK1/2 and PS in SH-SY5Y cells.

CONCLUSIONS/SIGNIFICANCE

Low levels of PS and its receptors suggest the participation of PS in some pathological changes in the brains of mdx mice.

Prosaposin-derived peptide alleviates ischaemia-induced hearing loss

CONCLUSION

An 18-mer peptide derived from the neurotrophic region of prosaposin (PS-pep) prevents hearing loss and cochlear damage due to transient cochlear ischaemia by activating an anti-apoptotic pathway. PS-pep is a potent candidate molecule for alleviating ischaemia-induced hearing loss.

OBJECTIVE

PS-pep was investigated for its protective effects against ischaemia-induced hearing loss and cochlear damage.

METHODS

Ischaemia was induced in both cochleae in Mongolian gerbils by pulling the ligatures around both vertebral arteries in an anterior direction using 5 g weights for 15 min. PS-pep was synthesized artificially and administered subcutaneously four times after the induction of transient cochlear ischaemia.

RESULTS

An increase in the auditory brainstem response threshold was alleviated in animals treated with 2.0 mg/kg PS-pep. Histological examinations conducted on day 7 showed that the loss of inner hair cells (IHCs) was more prominent than that of outer hair cells. Higher doses of PS-pep significantly alleviated IHC loss. An increase in the anti-apoptotic factor bcl-2 was also noted in the IHCs treated with PS-pep.

Experimental study of transient cochlear ischemia as a cause of sudden deafness

The etiology of sudden deafness or idiopathic sudden sensorineural hearing loss (ISSHL) remains unclear. Over the past 15 years, we have investigated the mechanisms of ischemic-induced hearing loss using a gerbil model of transient cochlear ischemia. In the gerbil, cochlear ischemia can be induced by occluding the bilateral vertebral arteries simultaneously at the neck, because the posterior communicating arteries of the Circle of Willis close spontaneously around 1 mo after birth. When 15 min ischemia was loaded on this animal, permanent hearing loss of about 25 dB and the death of hair cells, especially inner hair cells were induced. These pathological changes were mainly due to lack of an energy source, glutamate excitotoxicity, and the production of free radicals, especially superoxide and nitrous oxide species. Ischemic damage could be prevented by various procedures, such as cooling the cochlea, intratympanic administration of insulin-like growth factor 1 or AM-111 (an anti-apoptotic agent), and systemic administration of prednisolone (steroid), edarabone (free radical scavenger), ginsenoside Rb1 (Kanpo), hematopoietic stem cells, glia-cell derived neurotrophic factor, and liposome-encapsulated hemoglobin (artificial red blood cells). We also found that the cochlea was protected by the ischemic tolerance, indicating that minor cochlear ischemia alleviates or prevents inner ear damage in subsequent severe cochlear ischemia. As ISSHL usually occurs suddenly, with no preceding sign or symptom, we suggest that most ISSHL cases are caused by circulatory disturbance, probably at the stria vascularis.

Distribution of prosaposin in rat lymphatic tissues

Prosaposin (PSAP) is as a trophic factor and an activator protein for sphingolipid hydrolase in lysosomes. We generated a specific antibody to PSAP and examined the spatiotemporal distribution of PSAP-immunoreactive (PSAP-IR) cells in the lymphatic tissues of Wistar rats. Immunoblots of tissue homogenates separated electrophoretically showed a single band for PSAP in brain but two bands in spleen. PSAP-IR cells were distributed in both the red and white pulp of the spleen, in both the cortex and medulla of the thymus and in mesenteric lymph nodes. Many PSAP-IR cells were found in the dome portion of Peyer's patches and the number of PSAP-IR cells increased with the age of the rat. To identify the PSAP-IR cells, double- and triple-immunostainings were performed with antibodies against PSAP, CD68 and CD1d. The large number of double- and triple-positive cells suggested that antigen-presenting cells contained much PSAP in these lymphatic tissues. Intense expression of PSAP mRNA, examined by in situ hybridisation, was observed in the red pulp and corona of the spleen. In rats, the PSAP gene generates two alternative splicing forms of mRNA: Pro+9 containing a 9-base insertion and Pro+0 without the insertion. We examined the expression patterns of the alternative splicing forms of PSAP mRNA in the spleen. The presence of both types of mRNA (Pro+9 and Pro+0) indicated that the spleen contains various types of prosaposin-producing and/or secreting cells. These findings suggest diverse functions for PSAP in the immune system.

Severe vestibular dysfunction and altered vestibular innervation in mice lacking prosaposin

Prosaposin, a precursor of four glycoprotein activators (saposin A, B, C and D) for lysosomal hydrolases, has previously been shown to be important for normal adult cochlear innervation and the maintenance of normal hearing. In these studies, we now investigate prosaposin in normal vestibular epithelium and the functional impairment of balance caused by prosaposin ablation. In normal mice, prosaposin is localized to all 3 vestibular end-organs (ampullae, saccule, and utricle) and Scarpa's ganglion by RT-PCR, Western blot analysis and immunofluorescence. Ablation of prosaposin function caused severe vestibular dysfunction on a battery of behavioral tasks. Histologically, the KO mice demonstrated an exuberant cellular proliferation below the vestibular hair cells with disruption of the supporting cells. Electron microscopy further demonstrated inclusion bodies and cellular proliferation disturbing the normal neuroepithelial structure of the vestibular end-organs. Lastly, immunofluorescence (neurofilament 200 and synaptophysin) staining suggests that this cellular proliferation corresponds to afferent and efferent neuronal overgrowth. These data suggest that prosaposin plays a role not only in the maintenance of normal hearing but also an important role in the neuronal maturation processes of the vestibular sensory epithelium and the maintenance of normal vestibular system function.

Physiopathology of the cochlear microcirculation

Normal blood supply to the cochlea is critically important for establishing the endocochlear potential and sustaining production of endolymph. Abnormal cochlear microcirculation has long been considered an etiologic factor in noise-induced hearing loss, age-related hearing loss (presbycusis), sudden hearing loss or vestibular function, and Meniere's disease. Knowledge of the mechanisms underlying the pathophysiology of cochlear microcirculation is of fundamental clinical importance. A better understanding of cochlear blood flow (CoBF) will enable more effective management of hearing disorders resulting from aberrant blood flow. This review focuses on recent discoveries and findings related to the physiopathology of the cochlear microvasculature.

Chronological changes in prosaposin in the developing rat brain

Prosaposin is the precursor protein of four glycoproteins, saposins A, B, C, and D, which activate sphingolipid hydrolases in lysosomes. Besides this role, intact prosaposin is also known as a potent neurotrophic factor that prevents neuronal cell death and stimulates neurite outgrowth in in vivo and in vitro experiments. In the present study, we examined chronological changes in prosaposin immunoreactivity in the rat brain using immunofluorescence staining and Diaminobenzidine (DAB) immunohistochemistry. In the hippocampal regions CA1, CA3, and dentate gyrus, the strongest staining of prosaposin was observed on postnatal day 1. The prosaposin immunoreactivity then decreased gradually until postnatal day 28. But in the cerebral cortex, prosaposin staining intensity increased from postnatal day 1 to 14, then decreased until postnatal day 28. The prosaposin immunoreactivity co-localized with the lysosomal granules labeled by an anti-Cathepsin D antibody, indicating that prosaposin mainly localized in the lysosomes of the neurons. We also examined the chronological changes in prosaposin mRNA and its two alternatively spliced variants using in situ hybridization. We found that both the mRNA forms, especially the one without a nine-base insertion, increased significantly from embryonic day 15 to postnatal day 7, then decreased gradually until postnatal day 28. Abundant prosaposin expression in the perinatal stages indicates a potential role of prosaposin in the early development of the rat brain.

Expression patterns in alternative splicing forms of prosaposin mRNA in the rat facial nerve nucleus after facial nerve transection

Prosaposin acts as a neurotrophic factor, in addition to its role as the precursor protein for saposins A, B, C, and D, which are activators for specific sphingolipid hydrolases in lysosomes. In rats, the prosaposin gene generates two alternative splicing forms of mRNA: Pro+9 containing a 9-base insertion and Pro+0 without. The expression of these mRNAs changes after brain injury. We examined the expression patterns of the alternative splicing forms of prosaposin mRNA in the rat facial nerve nucleus for 52 days following facial nerve transection. Pro+0 mRNA increased within 3 days of transection, peaked after 5-10 days, and remained significantly elevated for 21 days. In contrast, the expression of Pro+9 mRNA was constant throughout the regenerative period. Prosaposin mRNA expression increased not only in facial motoneurons, but also in microglia during facial nerve regeneration. Our findings indicate that the saposin B domain of prosaposin, which is the domain affected by alternative splicing, plays an important role in both neurons and microglia during neuroregeneration.

Distribution of prosaposin in the rat nervous system

Prosaposin is the precursor of four sphingolipid activator proteins (saposins A, B, C, and D) for lysosomal hydrolases and is abundant in the nervous system and muscle. In addition to its role as a precursor of saposins in lysosomes, intact prosaposin has neurotrophic effects in vivo or in vitro when supplied exogenously. We examined the distribution of prosaposin in the central and peripheral nervous systems and its intracellular distribution. Using a monospecific antisaposin D antibody that crossreacts with prosaposin but not with saposins A, B, or C, immunoblot experiments showed that both the central and peripheral nervous systems express unprocessed prosaposin and little saposin D. Using the antisaposin D antibodies, we demonstrated that prosaposin is abundant in almost all neurons of both the central and peripheral nervous systems, including autonomic nerves, as well as motor and sensory nerves. Immunoelectron microscopy using double staining with antisaposin D and anticathepsin D antibodies showed strong prosaposin immunoreactivity mainly in the lysosomal granules in the neurons in both the central and peripheral nervous systems. The expression of prosaposin mRNA, examined using in situ hybridization, was observed in these same neurons. Our results suggest that prosaposin is synthesized ubiquitously in neurons of both the central and peripheral nervous systems.