OMgp! Molecules that make Neuron Growth a Nogo

Ask your doctor exactly what they are doing post-stroke to make sure your axons are growing and extending properly. No answer or a blank look is a reason to drop that doctor immediately because they obviously know nothing about  axon pathfinding and neurite outgrowth.
 You want to recover? Don't you? Then we are going to have to fire one hell of a lot of doctors until they actually know something about how to recover from a stroke.  This is all part of us paying it forward for future survivors.  If we have to fire a lot of doctors to accomplish that that is a small price to pay for something our doctors should have been doing all along. Eg. keeping up with the current stroke research. 

OMgp! Molecules that make Neuron Growth a Nogo

In the dark, cramped setting of the newborn brain, you slither along your guidepost. You have been slowly creeping on your chemically-determined path for what seems like ages wanting only to make a connection. As you stretch your greedy filopodia out, you hit something. An obstruction? A competing axon? Nothing obvious blocks you, yet you are suddenly forced backwards as your growth cone crumples. Only here do you finally identify the phantom forcing you back: the Sema3A clinging to your Plexin receptors.

Roman Giger, an associate professor and long-time researcher at University of Michigan, works to identify and understand properties of “phantom inhibitors” such as Sema3A. By using knockout mice and subsequent structural and functional analyses, his studies have illuminated a wide array of molecules that have the capacity to inhibit axon growth, dendritic growth, and synapse formation. There is nowhere near enough space to delineate all of Dr. Giger’s findings (seriously, he has had 32 publications in 12 years, take a look! https://www.ncbi.nlm.nih.gov/pubmed), so I will focus on only two here: Semaphorin5A and the Nogo-Receptors(NgRs).

When axon tracts in the CNS are damaged, several biological mechanisms swing into action to inhibit axon regeneration. Astrocytes will soon begin to aggregate and link tightly together to form a physical obstruction called a glial scar; this tissue bunch will also secrete an assortment of molecules (called myelin associated inhibitors, or MAIs) to chemically inhibit the busted axon from regrowing. For example, chondroitin sulfate proteoglycans (CSPGs), a mouthful of an MAI, will inhibit axon regeneration by binding to its receptor (RPTPsigma) on the axon membrane. Yet, knockout of RPTPsigma will incompletely disinhibit neurite growth from the injury site. This indicates that CSPG must bind to another partner to do its repulsive dirty work… which is where NgR1/2/3 comes in.

NgR1/NgR2/NgR3, three receptor subtypes found on the axon, are the well-known binding partners of an oligodendrocyte membrane protein and MAI that is aptly named Nogo. However, Dr. Giger noticed something strange: if you KO all three NgRs then axon regeneration is elevated, but if you KO these receptors in combinations (NgR1, or NgR2, or NgR1/NgR2, etc.), ONLY NgR1/NgR3 double-KO is sufficient to replicate the triple-KO’s level of regeneration. This implies that NgR1/3 inhibit neurite outgrowth by the same mechanism. Further studies revealed that NgR1/3 bind to CSPGs by a previously unknown binding site, mimicking RPTPsigma. When these three CSPG binding partners (RPTPsigma, NgR1, and NgR3) are KO’ed, crushed axons exhibit neurite outgrowth more extreme than even NgR1/3 double-KO; combined, these findings indicate that NgR1, NgR3, and RPTPsigma are all functionally redundant, and so are playing the same role in neurite inhibition.

Damaged axons are not the only target of molecularly-mediated growth inhibition. In his most recent study, Dr. Giger unveiled the ability of a membrane protein called Semaphorin5A (SEMA5A), to hinder excitatory synapse development. The Michigan scientist showed that when this molecule is KO’ed in mouse hippocampus, density of dendritic spines (where most excitatory synapses form) is significantly increased compared to controls (Figure 2a-g). Based on increase of PSD-95 (in excitatory synapses) but not gephryn (in inhibitory synapses) in KO animals, SEMA5A’s effects were determined to be excitation-specific.

With structure affected, function would logically be affected too, right? This is what Dr. Giger finds, by way of hippocampal patch clamp recordings- an increase in both the size and number of excitatory currents (called “mEPSCs”) entering hippocampal neurons in SEMA5A-KO mice (Figure 2h[left], 2i-k). These overly exuberant currents can be silenced using CNQX (an AMPA receptor blocker), evidencing that this excitatory receptor’s response is increased in the KO condition (Figure 2h[right]).

These findings aren’t just exciting for neuroscientists. SEMA5A is one of the SNPs (single nucleotide polymorphisms, or one-base genetic differences) seen in autism patients, which will surely catch the ear of clinicians; patients typically have a lower expression level of SEMA5A compared to those who are non-autistic (1,2). This would, following what Dr. Giger has found, indicate that autism patients should have more spines than normal, right? Elevated spine densities have been found in mouse models of autism, aligning perfectly with Giger’s findings(3). If SEMA5A-KO mice exhibit similar morphological phenotypes to autism mouse models, shouldn’t behavior be similar in these mice as well? It is: SEMA5A-KO mice show anti-social behavior, as exhibited by aversion to interaction with a “stranger” mouse, replicating what current autism mouse models have displayed(4). Dr. Giger’s SEMA5A-KO findings fit shockingly well with the current autism literature, implicating it as a strong candidate for further study of this disorder. SEMA5A research also shows us that stopping growth, in addition to initiating it, is essential for a healthy nervous system.

So, what’s stopping YOU? Roman Giger will be speaking at the Center For Neural Circuits and Behavior large conference room on October 28th, 2014 at 4:00 PM. His talk is entitled “Neural circuit assembly, plasticity, and repair in CNS health and injury”. Don’t inhibit yourself, come check it out!

 

Norah is a first-year student in the UCSD Neurosciences Graduate Program. She is currently rotating with Jared Young, working with a mouse model of bipolar disorder. She is adamant about psychiatric disorder research and has a penchant for bad puns.

 

1) Melin M, Carlsson B, Anckarsater H, Rastam M, Betancur C, Isaksson A, Gillberg C, Dahl N (2006). Constitutional downregulation of SEMA5A expression in autism. Neuropsychobiology 54:64-69.

2) Weiss LA, Arking DE, Daly MJ, Chakravarti A (2009). A genome-wide linkage and association scan reveals novel loci for autism. Nature 461:802-808.