Firoze B. Jungalwala

High-mobility-group (HMG) proteins are a family of non-histone chromosomal proteins which bind to DNA. They have been implicated in multiple aspects of gene regulation and cellular differentiation. Sulfoglucuronyl carbohydrate binding protein, SBP-1, which is also localized in the neuronal nuclei, was shown to be required for neurite outgrowth and neuronal migration during development of the nervous system. In order to establish relationship between SBP-1 and HMG family proteins, two HMG proteins were isolated and purified from developing rat cerebellum by heparin-sepharose and sulfatide-octyl-sepharose affinity column chromatography and their biochemical and biological properties were compared with those of SBP-1. Characterization by high performance liquid chromatography--mass spectrometry (HPLC-MS), partial peptide sequencing and western blot analysis showed the isolated HMG proteins to be HMG-1 and HMG-2. Isoelectric focusing, HPLC-MS and peptide sequencing data also suggested that HMG-1 and SBP-1 were identical. Similar to SBP-1, both HMG proteins bound specifically to sulfated glycolipids, sulfoglucuronylglycolipids (SGGLs), sulfatide and seminolipid in HPTLC-immuno-overlay and solid-phase binding assays. The HMG proteins promoted neurite outgrowth in dissociated cerebellar cells, which was inhibited by SGGLs, anti-Leu7 hybridoma (HNK-1) and anti-SBP-1 peptide antibodies, similar to SBP-1. The proteins also promoted neurite outgrowth in explant cultures of cerebellum. The results showed that the cerebellar HMG-1 and -2 proteins have similar biochemical and biological properties and HMG-1 is most likely identical to SBP-1.

Developmental expression of sulfoglucuronyl carbohydrate (SGC) and its binding protein, SBP‐1 was studied in the rat cerebral cortex to understand their function. Between embryonic day (ED) 14–19, SBP‐1 was strongly expressed in neurons of the ventricular zone and migrating neurons throughout the cortex. SBP‐1 declined at birth and by postnatal day (PD) 3 only the latest arriving neurons in the most superficial segment of the cortical plate expressed SBP‐1. Between ED 14–16, SGC was expressed in a thin row of glial cells near the ventricles and on their radial processes. Between ED 16–PD 3, SGC was not in neuronal cell soma, but was in neuronal plasma membranes and processes surrounding the neuronal perikarya. The expression of SGC declined similar to SBP‐1 and both of them disappeared by PD 7. The expression of SBP‐1 and SGC was chronologically coordinated with neuronal migration. SBP‐1 was specifically expressed in immature neuronal nuclei and plasma membranes. SBP‐1 and SGC were colocalized and were available for interaction with each other on neuronal cell membranes and processes. This was confirmed with isolated neurons in culture. As in vivo, the expression of SBP‐1 in neurons declined with time in culture. The dissociated cortical neurons when plated on SBP‐1 as a substratum produced extensive neuritic outgrowth. HNK‐1, anti‐SBP‐1 antibodies and sulfoglucuronyl glycolipid, SGGL specifically and severely reduced neurite outgrowth. SBP‐1‐SGC interactions provide a potential mechanism for guidance and cell signaling, in the processes of neuronal migration and terminal differentiation. J. Neurosci. Res. 62:186–205, 2000.

Sulfoglucuronyl carbohydrate (SGC) is expressed on several neural cell‐adhesion molecules and on glycolipids. SGC and its binding protein, SBP‐1 are developmentally regulated in the nervous system and have been implicated in regulating neurite outgrowth and cell–cell recognition during neuronal cell migration. To elucidate the role of interaction between SGC and SBP‐1, microexplant cultures of postnatal day 5 rat cerebellum were employed. In explant cultures, SGC was localized primarily in the neuronal cell processes, neurofilaments, and dendrites that emerge from the core of the explants up to 90 μm, after 24 hr in culture. SGC was also present in the short astrocytic processes near the core of the explant. SBP‐1 was localized mainly in the granule neuron cell bodies and faintly on cell plasma membranes and processes. Granule neurons, expressing SBP‐1, migrated outward in close contact with the SGC bearing neuronal processes, suggesting interaction between SGC and SBP‐1. The neurite outgrowth and cell migration were specifically and severely reduced, in dose‐dependent manners, by anti‐SGC (HNK‐1) and anti‐SBP‐1 antibodies and sulfoglucuronyl glycolipid (SGGL). Other irrelevant antibodies and glycolipids had little effect. The results showed that SBP‐1 was required for neurite outgrowth and that SGC‐SBP‐1 interaction was important for cell–cell recognition and cell migration. J. Neurosci. Res. 2:188–201, 2000