**Abstract**

Brain plasticity is regulated through dynamic interactions between perineuronal nets, matrix metalloproteases (MMPs) and the extracellular matrix (ECM). Several studies have identified a crucial role for vitamins D and B12 in brain development and a deficiency in these vitamins may contribute to the emergence of cognitive deficits, as well as the onset of both autism spectrum disorder and schizophrenia. However, the mechanisms underlying the interplay between ECM, MMPs, vitamins and these neuropsychiatric conditions are poorly understood. In this chapter, we seek to understand how the risk of neurodegeneration in vulnerable individuals and the aetiology of specific neuropsychiatric disorders are affected by vitamin D and B12 deficiency, in conjunction with low levels of the antioxidant glutathione, impaired GABAergic inhibition, and alterations in the permanent ECM.

**Keywords:** vitamin deficiency, perineuronal nets, matrix metalloproteases, parvalbumin interneurons, GABA, neurodevelopment

## **1. Introduction**

A proteoglycan-rich matrix, the perineuronal net (PNN) is a dense structure within the extracellular matrix (ECM), whose synapses form through gaps around many neuronal bodies and dendrites at a late stage in brain development. PNNs are formed at the end of a critical period of neurodevelopment, following the transformation of the central nervous system (CNS) from an environment conducive to neuronal growth and motility to one that is more restrictive, in response to several sensory inputs from both neurons and glia driving increased neuroplasticity [1]. The main components of the PNN matrix include several chondroitin sulfate proteoglycans (CSPGs), such as hyaluronan, link proteins, and tenascin-R and -C.

During mammalian development, hyaluronan binds to members of the lectican family originally produced in neurons, including versican V0 and V1 and neurocan [2], whereas aggrecan seems to be expressed by astroglial cells in the juvenile matrix [3]. Other lecticans include versican 2, brevican, phosphacan, tenascin-R and the

link proteins HAPLN2/Bral1 and HAPLN4/Bral2 are only observed in more mature matrix environments approximately 2 weeks after birth [4–6], in contrast to the composition of the juvenile matrix. Following this period, shifts in brevican expression occur at the end of myelination, leading to white-matter precursor changes from an oligodendroglial to an astrocytic lineage [7], and resulting in a compact extracellular matrix forming the PPN [8].

PNNs have been observed 2–5 weeks after birth around parvalbumin (PV<sup>+</sup> ) expressing GABAergic interneurons in pyramidal cortex, and around large motor neurons of the brainstem and spinal cord. This period coincides with the end of experience-dependent refinement of the synaptic network [8], but marked by a still critical period of matrix turnover and proteoglycan degradation by ADAMTS metalloendopeptidases and matrix metalloproteinases (MMPs) [8, 9]. PNN formations can also be observed in several distinct areas of the CNS, such as other regions of the cerebral cortex, the hippocampus (HPC), thalamus, and cerebellum [8].

PNNs in the adult CNS secrete hyaluronan through the action of membranebound HA synthase, an enzyme linked to the action of link proteins, lecticans, tenascin-R and chondroitin sulphate proteoglycans (CSPGs), creating supramolecular aggregates on the surface of neurons [1]. Other relevant glycoproteins besides CSPGs include Reelin, mainly secreted by Cajal–Retzius cells and involved in the control of neuronal migration and the establishment of cell aggregation and dendrite formation during the embryonic and early postnatal stages of development [10]. In adulthood, Reelin signalling is involved in the modulation of synaptic function and binds to very-low-density lipoprotein receptors and apolipoprotein E receptor 2 [11]. Increased clustering of Reelin receptors leads to a build-up of DAB1 proteins on the neuron membrane, greater activation of Src/SFK family kinases, and tyrosine phosphorylation of N-methyl-D-aspartate receptors (NMDARs), resulting in a net increase of receptor activity (**Figure 1**) [12]. Reelin insufficiency may lead to alterations in NMDAR clustering and LTP, such as in dysfunctional GABA-ergic transmission in the cerebral cortex and hippocampus observed among the morphofunctional signalling changes in schizophrenia (SZ) [12].

In fact, alterations of GABAergic signalling within a prenatal stress period have been identified as important factors in the development of SZ [13], autism spectrum disorder (ASD) [14], and epilepsy [15], often leading to an altered density of GABAergic cells and aberrant oscillatory activity. However, one functional model of brain development has proposed that prenatal stress involves DNA methylation, possibly inducing methylation of the gene responsible for Reelin promoter, with the consequent down-expression of Reelin resulting in abnormalities within the neuronal architecture of the prefrontal cortex, a reduction in dendritic complexity and a decreased number of GABAergic neurons, leading to altered developmental neuronal connectivity [13].

Animal studies have demonstrated that DNA methylation in the BDNF gene controls its expression during forebrain development in mice [16]. Furthermore, binding of BDNF and nerve growth factor (NGF) neurotrophins to their respective receptors (TrkB/A) triggers the PI-3kinase/AKT pathway, with activation of the mammalian target of rapamycin (mTOR) [17] and Akt-dependent inhibition of the serine/threonine kinase Gsk3β, resulting in decreased transcription of pro-inflammatory cytokines (IL-1β, TNF-α, and IL-6) [18]. Since Reelin/lipoprotein receptors do not contain a cytoplasmic kinase domain, the core Reelin signalling pathway seems to be associated with tyrosine kinase receptor (RTK or Trk) activity [19], the most likely coreceptor candidate [20]. As such, Reelin signalling from the ECM in collaboration with TrkB/A receptor activation, leads to increased phosphorylation of *Vitamins D and B12, Altered Synaptic Plasticity and Extracellular Matrix DOI: http://dx.doi.org/10.5772/intechopen.100055*

#### **Figure 1.**

*Effect of Reelin concentrations in the ECM on NMDA signalling. Reelin activates adaptor protein disabled 1 (DAB1) by binding with its very-low-density lipoprotein receptor (VLDLR) and apolipoprotein E receptor type 2 (APOER2). DAB1 is phosphorylated by Src family kinases (SFK) at different sites on the protein - this phosphorylation occurs mainly through the action of a co-receptor, tyrosine kinase receptor (RTK or Trk), implicated in a variety of cellular processes including growth, differentiation, and regulation of energy metabolism in the neuron. DAB1 phosphorylation leads to inhibition of the serine/threonine kinase Gsk3β via protein kinase B (Akt), where a decrease in AKT phosphorylation levels with subsequent high levels of GSK-3*β *phosphorylation, has been observed in lymphocytes and brains of individuals with schizophrenia (SZ). Clustering of Reelin receptors via SKF activation also leads to greater tyrosine phosphorylation of N-methyl-D-aspartate receptors (NMDARs), resulting in a net increase of receptor activity following the induction of long-term potentiation via Ca2<sup>+</sup> regulation. This signalling cascade appears to be an essential process for neurobiological regulation during neurodevelopment (modified from [12]). Created with BioRender.*

#### **Figure 2.**

*Hypothetical signalling cascades for GSK3β modulation of the expression of pro-inflammatory and antiinflammatory cytokines in glial cells. Receptor crosstalk between receptors TrkB/A and Reelin in the ECM (see Figure 1), increase the phosphorylation of serine-9 GSK3β leading to GSK3β inhibition, an increase in the translocation of CREB from the cytoplasm to the nucleus, and an increase in the transcription of antiinflammatory cytokine (IL-10). In fact, GSK3β can modulate the expression of both pro-inflammatory and anti-inflammatory cytokines. GSK3β activation in glial cells triggers the NF-*κ*B pathway and the translocation of NF-*κ*B from the cytoplasm to the nucleus, leading to an increase in the transcription of pro-inflammatory cytokines ((IL-1*β*, TNF-*α*, and IL-6), via the action of CREB-binding protein (CBP). Inhibition of GSK3β results in an increase of CREB translocation from the cytoplasm to the nucleus. Phosphorylated CREB binds specifically to the nuclear CBPs at transcriptional sites, resulting in increased transcription of anti-inflammatory cytokines such as IL-10 (modified from [21]).*

serine-9 GSK3β, with the inhibition of GSK3β in glial cells and leukocytes resulting in more CREB being translocated from the cytoplasm to the nucleus, and an increase in the transcription of anti-inflammatory cytokines (IL-10) (**Figure 2**) [21].

PNN development and the maturation of PV<sup>+</sup> inhibitory cells, as well as processes such as myelination, mark the end of the critical period of human neurodevelopment [22]. Disruption or delay to the formation of the PNN results in the resumption or extension of the time window for neuroplasticity in the brain [23], wherein the nervous system is more sensitive to epigenetic, physical, biochemical, environmental, and nutritional factors. The effects of nutrition on individuals during gestational and early development have been extensively researched, leading many researchers to conclude that nutritional factors such as vitamins, folate and iodine can cause long-lasting impacts in neurodevelopment [24, 25]. As the foetus' and newborn's acquisition of vitamins like B12 and D, depends to a great extent on maternal diet, such research has increasingly focussed on the impact of the mothers' vitamin deficiency on their offspring's brain development during the foetal and exclusive breastfeeding stages.

S-adenosyl methionine (SAM) is a universal methyl donor for some of the main methylation reactions. Vitamin B12 is an important cofactor in the one-carbon cycle

#### *Vitamins D and B12, Altered Synaptic Plasticity and Extracellular Matrix DOI: http://dx.doi.org/10.5772/intechopen.100055*

and is involved in the formation of SAM. Vitamin B12 supplements have been shown to improve pregnancy outcomes and reduce the risk of neurodevelopmental disorders in the developing child [26]. In rats, dose-dependent vitamin B12 supplementation was able to maintain the levels of docosahexaenoic acid (DHA) and BDNF in the hippocampus and cortex in pups at birth, and BDNF in the hippocampus at 3 months of age [17]. In addition, the combination of omega-3 fatty acid and vitamin B12 administration maintained spatial memory performance in neonates [17]. Experimental evidence suggests that DHA, together with greater levels of physical exercise, increases activated forms of CREB and synapsin I, reducing oxidative stress in the hippocampus [18].

Vitamin D deficiency may also reduce the integrity of PNNs and synaptic plasticity in neuropsychiatric disorders through the modulation of MMPs. Vitamin D deficiency has been associated with vulnerability to SZ [27], as well as ASD [28] and attention deficit and hyperactivity disorder (ADHD) [29], the two most common neurodevelopmental disorders. As mentioned earlier, ADAMTS and MMPs are two families of endogenous zinc-dependent proteases, secreted as inactive proenzymes that cleave ECM components. Alterations in the genes that encode MMP-16 and MMP-9 have been observed in patients with SZ [30]. High levels of MMP-9 can support the proteolytic cleavage of ECM with permissive synaptic plasticity but also lead to abnormal aggrecan degradation, abnormal development and neural excitability [30]. Chronic stress and neurological trauma can enhance MMP-9 levels in the brain [31, 32], and consequently raise the risk of SZ. A plausible proposal has been made that vitamin D deficiency leads to PNN degradation in patients with SZ [27]. In fact, vitamin D

#### **Figure 3.**

*Vitamin D deficiency and PNN formation during neurodevelopment. The figure above shows a neuron enveloped by a PNN. Vitamin D deficiency may induce a deficit in ECM organisation over the course of neurodevelopment, leading to the PNN loss and network-wide dysfunction in GABAergic, glutamatergic, and dopaminergic neurotransmission. Vitamin D deficiency is also linked to altered transcription of calcium channels (L-VGCC) potentially increasing the level of calcium input into the neuron, and to altered neuronal nitric oxide synthase (nNOS) activity, resulting in an increase of nitric oxide (NO) secretion into the extracellular space and elevated levels of MMP-9. This enhanced MMP-9 expression induces increased aggrecan synthesis, resulting in disruptions to the network of several neurotransmission systems important for normal cognitive function (spatial learning deficits), and SZ, autism and ADHD. Abbreviations: L-VGCC: L-type voltage-gated calcium channel; ECM: Extracellular matrix; MMP-9: Matrix metalloproteinase-9; nNOS: Neuronal nitric oxide synthase; NO: Nitric oxide; PNN: Perineuronal net; SZ: Schizophrenia; ADHD: Attention deficit and hyperactivity disorder (modified from [34]).*

deficiency is associated with increased MMP-9 production [33] and calcium activity on the neuronal membrane, leading to increased nitric oxide (NO) formation and higher MMP-9 levels, and further appears to modulate its endogenous inhibitor TIMP1 (tissue inhibitor of MMP) [34]. Aggrecan-rich PNNs undergo restructuring leading to the occurrence of more synaptic anomalies and greater network dysfunction in GABAergic, glutamatergic, and dopaminergic neurotransmission, as evidenced by some forms of SZ (**Figure 3**) [34]. Cognitive deficits, such as spatial learning deficits, have been observed in adult mice with vitamin D deficiencies, with reduced density of PNNs and neural networks within the hippocampus [35].
