Oculocereborenal Syndrome of Lowe (OCRL): Potential Involvement of Disturbances in Megalin Fnxn's & Relevance to the Effects of Phosphate Depletion

(I meant to type oculocerebrorenal in the title, but my mind isn't working especially well this week, due to the intranasal flu vaccine.) This article [Ramanathan et al., 2009: (http://www.sajaa.co.za/index.php/sajaa/article/viewPDFInterstitial/387/428)] is maybe a poorly-chosen example of an article about Lowe syndrome [the "oculocerebrorenal syndrome of Lowe" (OCRL)], but there are plenty of articles on it (http://scholar.google.com/scholar?q=oculocerebrorenal+Lowe&hl=en). On the surface, the manifestations of OCRL look very similar to Fanconi's syndrome and to the manifestations of intracellular phosphate depletion, but, apparently, many people who have OCRL, caused by hypofunctionality of the OCRL protein(s), or who have a subtype of Dent's disease that is caused by mutations in the OCRL protein (the OCRL protein is an inositol polyphosphate 5-phosphatase that hydrolyzes mainly phosphatidylinositol 4,5-bisphosphate, or PtdIns(4,5)P2, into phosphatidylinositol 4-phosphate but also hydrolyzes other phosphatidylinositols, including phosphatidylinositol 3,4,5-triphosphate, or PtdIns(3,4,5)P3, into phosphatidylinositol 4,5-bisphosphate, etc.) do not display the phosphaturia or hypophosphatemia and rickets that characterize X-linked hypophosphatemic rickets and that occur or can occur in Fanconi's syndrome [Kleta, 2008: (http://www.ncbi.nlm.nih.gov/pubmed/18667737)]. But there's still a lot of overlap with the manifestations of intracellular phosphate depletion and with Fancon's syndrome and other causes of hypophosphatemia. There's hypercalciuria, proteinuria, hypercalciuria, etc., and the proximal tubules are relatively selectively affected, in terms of the effects of the disorder on the kidneys. OCRL also causes vacuolar changes in myelin that fall short of overt demyelination, and OCRL causes cataracts and glaucoma, etc. The encephalopathy and mental retardation in OCRL is seemingly more severe than the effects of hypophosphatemia, but I've cited articles in past postings showing that rather devastating central nervous system damage can result from hypophosphatemia. It's conceivable that intracellular phosphate depletion in neurons and astrocytes, in parts of the brain, is more common than is recognized and that the depletion of phosphate does, in fact, commonly cause vacuolar myelopathy (which, incidentally, is not a very specific neuropathological change and occurs in a variety of contexts, including vitamin B12 depletion, etc.). It's not all *that* generic, as a neuropathological manifestation, however, and OCRL is still similar to Fanconi's syndrome, in my opinion and the opinion of others [Erdmann et al., 2008: (http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2025683)(http://www.ncbi.nlm.nih.gov/pubmed/17765681)]. It's not really useful to say that it's a totally different condition, when there's so much overlap of the manifestations.

My first thought was that the mutations in OCRL were causing intracellular phosphate depletion with something like intermittent hypophosphatemia, but the most direct manifestation of OCRL (the syndrome) is the aberrant accumulation of PtdIns(4,5)P2, primarily. One possibility is that, in humans who display significant intracellular phosphate depletion and develop proximal tubular acidosis and other changes that overlap with those found in OCRL or Dent's disease caused by mutations in the OCRL protein (Dent's disease can also result from mutations in chloride transporters), there's a generalized depletion of different phosphatidylinositols that, by some mechanism, leads to a similar skewing of the abundances of different phosphatidylinositols in favor of PtdIns(4,5)P2, as in OCRL. Or it's possible that, in OCRL, the impairment in the hydrolysis of PtdIns(4,5)P2 leads to a reduction in the availability of inorganic phosphate for use in ATP formation, much as the sequestration of phosphate in fructose 2,6-bisphosphate can cause ATP depletion following a fructose load. Or, maybe the formation or turnover of mutiple PtdIns's are upregulated in a way that sequesters inorganic phosphate, in multiple pools of PtdIns's, in a way that's maladaptive and detrimental to energy metabolism. Or, it may have nothing to do with energy metabolism.

But Erdmann et al. (2008) found, basically, that the accumulation of PtdIns(4,5)P2 in endocytic vesicles apparently deranged the trafficking of megalin to the apical membranes of proximal tubule epithelial cells. In any case, there can be abnormalities in the uptake of calcium and other constituents of tubular fluid and in receptor-mediated endocytosis by megalin in response to the conditions that occur in people who have OCRL mutations (and the OCRL syndrome) (Erdmann et al., 2008). Erdmann et al. (2008) mention that deranged megalin signalling could account for the CNS abnormalities and that patients with Dent's disease (even the forms due to mutations in chloride transporters) exhibit abnormalities in the functioning of megalin. Erdmann et al. (2008) didn't mention it, but megalin transports vitamin B12 bound to transcobalamin (http://scholar.google.com/scholar?hl=en&q=transcobalamin+megalin) and also transports vitamin D bound to vitamin D binding protein (http://scholar.google.com/scholar?hl=en&q=%22vitamin+D+binding+protein%22+megalin). Megalin serves a transport function across the blood-brain and blood-CSF barriers (http://scholar.google.com/scholar?hl=en&q=megalin+%22blood-brain%22+OR+%22blood-CSF%22), and the "spongy" changes in myelin or "pallor" of myelin (http://scholar.google.com/scholar?hl=en&q=myelin+OCRL+pallor+OR+spongy) seen in people who have OCRL hypofunctionality are reminiscent, in my mind, of the vacuolar myelopathy seen in subacute combined degeneration, due to vitamin B12 depletion (http://scholar.google.com/scholar?hl=en&q=vacuolar+myelopathy+B12), or in humans who have methionine adenosyltransferase deficiency, etc. [(http://scholar.google.com/scholar?hl=en&q=vacuolar+myelopathy+methionine+OR+%22S-adenosylmethionine%22+OR+%22S-adenosyl-L-methionine%22); (http://scholar.google.com/scholar?hl=en&q=myelin+deficiency+%22methionine+adenosyltransferase%22)].

It's interesting that cycloleucine, an inhibitor of methionine adenosyltransferase (MAT), the enzyme that synthesizes S-adenosylmethionine (SAM-e), causes "vacuolation" of myelin (http://scholar.google.com/scholar?hl=en&q=myelin+cycloleucine+vacuolation+OR+vacuolar). It's likely that "vacuolar myelopathy," which can sometimes be characterized by pathological changes in the myelin and also in oligodendrocytes or other cells [such as inclusion bodies in the nuclei of different cell types (http://scholar.google.com/scholar?hl=en&q=%22vacuolar+myelopathy%22+inclusion+body+vacuolation+OR+vacuolar)], is heterogeneous, but one interpretation would be to say that phosphate depletion can reduce SAM-e levels by reducing ATP and adenosine nucleotide pools in oligodendrocytes and other cell types. ATP depletion is known to be capable of causing SAM-e depletion [see either Morrison et al., 1997, or Eto et al., 2002, both of whom showed that SAM-e levels were decreased in the brains of people who had had Alzheimer's disease (the authors in at least one group were saying, correctly, in my view, that the SAM-e depletion was really likely to have been caused by ATP depletion): (http://scholar.google.com/scholar?hl=en&q=ATP+%22severely+decreased%22+Alzheimer%27s+%22S-adenosylmethionine%22)], and that, together with derangements in the abundance of PtdIns(4,5)P2 and other phosphatidylinositols (causing reduced vitamin B12 transport into the brain by reducing the megalin-mediated uptake of B12, etc.), could account for the web of associations I've discussed in this article. It's interesting that Reed et al. (2007) [Reed et al., 2007: (http://www.ncbi.nlm.nih.gov/pubmed/17392004)] found that cats that displayed low serum vitamin B12 and low serum folate levels also tended to display low serum phosphate levels (Reed et al., 2007). One could attribute that to any number of changes and say that the cats had some kind of Fanconi's syndrome that impaired reabsorption of folate binding protein, transcobalamin, and also inorganic phosphate from the tubular fluid. It's known that megalin knockout mice display low-molecular weight proteinuria (http://scholar.google.com/scholar?hl=en&q=megalin+proteinuria), as discussed by Erdmann et al. (2008), and lose different vitamins and other proteins in their urine, and megalin also transports folate binding protein (http://scholar.google.com/scholar?hl=en&q=megalin+folate+binding+protein). But phosphate depletion per se can cause metabolic acidosis or ATP depletion without acidosis in the proximal tubules and could, in my opinion, be a cause and consequence of proximal tubule pathologies. Here's another article that describes an association of B12 depletion with phosphate depletion and that could be explained by the fact that malabsorption, as in liver disease, can cause hypophosphatemia and cobalamin deficiency and also folate depletion [Wojtyczka, 1998: (http://cs.portlandpress.com/cs/095/0735/cs0950735.htm)(http://www.ncbi.nlm.nih.gov/pubmed/9831699)]. Those types of effects, such as loss of vitamin B12 and reduced folates and phosphate in the urine, could explain some of the post-infectious mono issues that people have [(http://scholar.google.com/scholar?hl=en&q=infectious+mono+nephritis+OR+tubular+OR+tubulointerstitial+OR+%22proximal+tubule%22); (http://scholar.google.com/scholar?hl=en&q=infectious+mono+adverse+OR+complication)]. It could be similar to the research showing that cerebral folate deficiency can result from expansion of the pools of antibodies that bind to the reduced folate carrier and other folate transporters at the blood-CSF barrier, given that the immune infiltration of the EBV-infected proximal tubule epithelial cells (http://scholar.google.com/scholar?hl=en&q=infectious+mono+EBV+%22proximal+tubule%22) could create a mess of immune-mediated impairments in proximal tubule functioning (such as by cytokine-mediated disturbances in energy metabolism, etc.). Supposedly, EBV doesn't infect choroid plexus epithelial cells, but I wouldn't be surprised if it did (http://scholar.google.com/scholar?hl=en&q=EBV+%22choroid+plexus%22). There are some significant problems with the notions that a lot of people have about the cell types that EBV supposedly can or can't infect. Here are some more searches [(http://scholar.google.com/scholar?hl=en&q=%22choroid+plexus%22+CD21+OR+C3d+OR+C3R); (http://scholar.google.com/scholar?hl=en&q=EBV+C3R+OR+C3d+OR+CD21)]. Everyone assumes that CD21 isn't likely to be expressed by cells in the CNS and that EBV must infect cells by binding to CD21, but what if it isn't true. A lot of viruses can infect cells using multiple transport mechanisms, some of which have only recently been discovered for influenza, for example. Also, there are significant problems with detecting EBV proteins during autopsies, and many articles look for EBV DNA or viremia (there's not going to be a bunch of viral DNA floating around, all over the place, in a cell latently-infected with EBV). EBV infects epithelial cells in basically every other organ, and it probably infects astrocytes and microglia (http://scholar.google.com/scholar?hl=en&q=EBV+infection+astrocyte+OR+%22human+monocytes%22) and pericytes (http://scholar.google.com/scholar?hl=en&q=resident+macrophage+pericyte+brain) and fibroblasts (i.e. meningeal fibroblasts, probably) [see Koide et al., 1997: (http://scholar.google.com/scholar?hl=en&q=EBV+fibroblasts)]. That type of effect on the proximal tubules could reduce phosphate and vitamin D and reduced folate and vitamin B12 reabsorption by the kidneys and could produce similar impairments at the blood-CSF barrier, etc. (http://scholar.google.com/scholar?hl=en&q=infectious+mono+brain+complication+OR+adverse).

I don't claim to be able to explain all of the different manifestations of these conditions, but the overlap of the effects of OCRL mutations with the effects of idiopathic Fanconi's syndrome and also the effects of intracellular phosphate depletion are fairly difficult to ignore completely. There must be some explanation, but it's interesting, in any case.

Epstein-Barr Virus Infection of Astrocytes and Monocytes: Potential Relevance to Research on Multiple Sclerosis and Astrocyte Cell Cycle Re-Entry

These articles [Chaudhuri, 2005: (http://www.ncbi.nlm.nih.gov/pubmed/15617877); Behan et al., 2002: (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=Behan+Chaudhuri+Roep+%22THE+PATHOGENESIS+OF+MULTIPLE+SCLEROSIS+REVISITED%22); VanAmerongen et al., 2004: (http://www.direct-ms.org/pdf/VitDMS/VanAmerongenVitDMSreview.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/15054436); Cepok et al., 2005: (http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1077174)(http://www.ncbi.nlm.nih.gov/pubmed/15841210); Diesel et al., 2005: (http://clincancerres.aacrjournals.org/cgi/content/full/11/15/5370)(http://www.ncbi.nlm.nih.gov/pubmed/16061850); Sanders et al., 1996: (http://www.ncbi.nlm.nih.gov/pubmed/8799216); Prokova et al., 2002: (http://www.jbc.org/cgi/content/full/277/11/9342)(http://www.ncbi.nlm.nih.gov/pubmed/11781310?dopt=Abstract); Koch et al., 2006: (http://www.ncbi.nlm.nih.gov/pubmed/17050217)] are really good, and Chaudhuri (2005) suggests that vitamin D repletion during brain development may protect against abnormal astrocyte apoptosis later in life and thereby confer protection against multiple sclerosis. This is interesting and is similar to the vitamin D hypothesis of schizophrenia [McGrath and colleagues: (http://scholar.google.com/scholar?q=%22vitamin+D%22+schizophrenia&hl=en&lr=)], in the sense that there's this concept of vitamin D deficiency, during development, creating abnormalities in brain development that do not manifest themselves until relatively later in life than one might expect them to. For example, vitamin D depletion during brain development drastically decreases the expression and protein content of the low-affinity neurotrophin receptor (p75NTR), which binds all of the neurotrophins and plays crucial roles in the regulation of not only apoptosis or protection against apoptosis, by NGF and other neurotrophins (NT-3, NT-4, BDNF, etc.), but in the trophic effects of NGF in the adult brain.

Holmoy (2008) suggested that vitamin D repletion could protect against brain damage due to late Epstein-Barr Virus (EBV) infection (i.e. after early childhood, when infection is often asymptomatic or less destructive to the brain), which tends to produce an expansion of autoreactive T-cell populations [Holmoy, 2008: (http://www.ncbi.nlm.nih.gov/pubmed/17574770)]. There's actually research showing that EBV can infect astrocytes [Menet et al., 1999: (http://www.ncbi.nlm.nih.gov/pubmed/10438862)] and monocytes and other cells of the monocyte-macrophage lineage [Savard et al., 2000: (http://www.ncbi.nlm.nih.gov/pubmed/10684275); (http://scholar.google.com/scholar?num=100&hl=en&lr=&cites=16392233215499070431)], which means that EBV may very well infect microglia and perivascular macrophages, etc. There still seems to be a popular sentiment that EBV only infects B-cells and epithelial cells, but there is overwhelming evidence that this is not the case and that EBV infects cells in the brain en masse during infectious mononucleosis (the term mononucleosis refers to the characteristic finding of monouclear phagocyte, or monocyte, infiltration of tissues infected by EBV; most cases of infectious mono are the result of primary EBV infection, although some can be from primary CMV infection or EBV infection that causes polyclonal, EBV-infected B-cells to start producing anti-CMV IgM and make it look like a person who had previously been infected with CMV has a primary CMV infection). I don't feel like going through papers and discussing them, but here are some hastily-done searches showing vast numbers of articles on the subject (http://scholar.google.com/scholar?num=100&hl=en&lr=&q=mononucleosis+brain+OR+encephalitis+OR+encephalopathy+OR+meningitis+OR+meningeoencephalitis); (http://scholar.google.com/scholar?q=mononucleosis++brain+OR+encephalitis+OR+encephalopathy+OR+meningitis+OR+meningeoencephalitis&num=100&hl=en&lr=&scoring=r&as_ylo=2004)]. The pro-inflammatory response during infectious mono is massive, and the notion that the blood-brain barrier would be impermeable to infiltration by EBV-infected, polyclonal B-cells is not reasonable. Also, an important distinguishing feature of infectious mono is enlargement or lymphadenopathy in the posterior cervical lymph nodes that provide lymphatic drainage to the brain, producing a stiff neck, etc. The oligoclonal IgG antibodies in the CSF of people with multiple sclerosis have repeatedly been shown to bind EBV proteins (Cepok et al., 2005), and Cepok et al. (2005) go into all the research showing that type of thing. It's possible that the immune response is being directed against other latently-infected B-cells, etc., but the evidence is pretty substatial that late EBV infection plays some role in the etiology of multiple sclerosis, in my opinion. To think that astrocytes and probably microglia and other cell types in the central nervous system would be spared infection makes no sense to me. So it probably occurs in many or most people who are infected with EBV (90-95 percent of the US population, by age 26-27), and one might look for some differences in the degree of ongoing damage or in the pattern of gene expression by EBV (i.e. the latency pattern) in astrocytes or microglia, etc. (discussed below) of people who go on to develop multiple sclerosis, in comparison to controls.

Behan et al. (2002) discuss a lot of evidence that inappropriate astrocytic cell-cycle re-entry plays a prominent role in the etiology of multiple sclerosis, and the authors, one of whom is Chaudhuri (see Chaudhuri, 2005), also discuss the association of multiple sclerosis with glioblastoma multiforme and with rare, diffuse forms of gliomas, etc. That article is superb and is really brilliant, and yet it's not even indexed in Medline. The fact that vitamin D analogs have been used to treat glioblastoma multiforme is interesting, and the effects of vitamin D receptor (VDR) ligands, including calcitriol itself, on the astrocytic cell cycle could suggest that they could protect against astrocytic cell cycle re-entry and apoptosis in people with multiple sclerosis. I tend to think they wouldn't be all that effective in that regard and that the focus of Chaudhuri (2005) on the developing brain makes more sense. But the focus on astrocytes (Chaudhuri, 2005; Behan et al., 2005) is really intriguing, and it suggests to me that other measures might protect against abnormal astrocyte proliferation and apoptosis (i.e. guanosine and other intravenously-administered purine nucleotides or those in combination with energy substrates, etc.). That's just my opinion. It's interesting that VDR activation leads to very complex interactions with the transforming growth factor-beta signalling cascade, such as by forming heterodimers with Smad3 and potentiating many Smad3-induced transcriptional changes (VanAmerongen et al., 2004), and that the EBV latent membrane protein-1 suppresses Smad3-dependent transcriptional changes (Prokova et al., 2002). Smad3 is phosphorylated by type I TGFbeta receptors and is thereby activated as a transcription factor. Smad3 interacts with many proteins, but the suppression by LMP1 of the TGFbeta-induced and Smad3-mediated increase in p21WAF1/Cip1 expression (Prokova et al., 2002) is a relatively specific intersection with the transcriptional program that tends to be induced by VDR activation. The p21WAF1/Cip1 gene is a major cell-cycle-regulatory gene whose expression is responsive to and increased by VDR activation. The gene product allows for enhanced DNA repair before cell division, etc., and contributes to the antiproliferative and differentiating effects of VDR activation. That's just one example, but it lends credence to the hypothesis of Holmoy (2008) and suggests that the interactions of VDR-ligand-induced transcriptional changes with EBV-induced transcriptional changes may be relatively direct and may go beyond the realm of VDR-ligand-induced increases in interleukin-10 output from monocytes, etc. It might be possible to look for the effects of vitamin D or its analogs on EBV-infected, cultured monocytes or astrocytes or to look for associations of 25-hydroxyvitamin D levels with the incidences of glioblastoma among patients with multiple sclerosis? That sounds pretty difficult. There are some recent articles discussing all the problems with detecting herpesviruses in the brain during autopsies. Serafini et al. (2007) [Serafini et al., 2007: (http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2118531)(http://www.ncbi.nlm.nih.gov/pubmed/17984305)] found that cells in perivascular regions of the brains of people with multiple sclerosis were immunoreactive for LMP1 and other latency-associated EBV proteins, and it doesn't sound like that can be casually attributed to infiltrating, EBV-infected B-cells, etc. Someone could look for an association between 25-hydroxyvitamin D levels at death and the latency pattern of EBV infection in the brains of people with MS (or just look for different latency patterns in people with MS). I'm just thinking out loud with this.